There is no official documented way to add another physical storage to an existing APFS container.

I tried several possibly "undocumented" apfsVerbs like addPhysical, addPhysicalStorage or addStorage without success.

The feature may still be implemented later, since creating APFS FusionLCs is possible already:

diskutil ap createContainer [-main] device [-secondary] [device]

Example:

$ diskutil ap createContainer disk1s2 disk2s2    
Creating container with disk1s2 disk2s2
Started APFS operation on disk1s2 Data
Creating a new empty APFS Container
Unmounting Volumes
Switching disk1s2 to APFS
Switching disk2s2 to APFS
Creating APFS Container
FusionLC autodetect: LC Fusion
Created new APFS Container disk4
Disk from APFS operation: disk4
Finished APFS operation on disk1s2 Data

$ diskutil ap addVolume disk4 APFS Data
Exporting new unencrypted APFS Volume "Data" from APFS Container Reference disk4
Started APFS operation on disk4
Preparing to add APFS Volume to APFS Container disk4
Creating APFS Volume
Created new APFS Volume disk4s1
Mounting APFS Volume
Setting volume permissions
Disk from APFS operation: disk4s1
Finished APFS operation on disk4

Again, yes, you have deleted all the local snapshots. But you clearly have an external Time Machine drive. And Time Machine snapshots (local or remote) place certain requirements on the source drive. From man tmutil:

BACKUP STRUCTURE
     Throughout this manual, specific language is used to describe particular
     "realms" associated with Time Machine backups. It is important to under-
     stand this terminology to make effective use of tmutil and its manual.

     backup source
             A volume currently being backed up by Time Machine.

     backup disk
             The HFS+ volume that contains Time Machine backups.

     backup destination
             In the case of a local destination, a synonym for backup disk.
             For network destinations, this is the AFP or SMB share on which
             the backup disk image resides.

     backup disk image (or backup image)
             A sparsebundle that, when mounted, is the backing store for a
             volume that is a backup disk.

     backup store
             The top-level "Backups.backupdb" directory at the root of a
             backup disk. E.g.,

             /Volumes/Chronoton/Backups.backupdb

     machine directory
             A directory inside a backup store that contains all the backups
             for a particular computer. For local destinations, a backup store
             can contain multiple machine directories, all for separate com-
             puters. E.g.,

             /Volumes/Chronoton/Backups.backupdb/thermopylae

--

Remember the error?

E.g., (perhaps caused by APFS Snapshot usage by Time Machine)?

Read this one:

     snapshot
             A directory inside a machine directory that represents a single
             initial or incremental backup of one computer. The word "snap-
             shot", in most contexts, is a generic term and is not to be con-
             fused with a "local Time Machine snapshot", which is simply a
             snapshot stored locally on the computer. E.g.,

             /Volumes/Chronoton/Backups.backupdb/thermopylae/2011-07-03-123456

     snapshot volume
             A directory inside a snapshot that represents a single initial or
             incremental backup of one backup source. E.g.,

             /Volumes/Chronoton/Backups.backupdb/thermopy-
             lae/2011-07-03-123456/Mac HD

VERBS
     Each verb is listed with its description and individual arguments.

     destinationinfo [-X]
             Print information about destinations currently configured for use
             with Time Machine. For each backup destination, the following
             information may be displayed:

             with Time Machine. For each backup destination, the following
             information may be displayed:

                 Name          The volume label as shown in Finder.
                 Kind          Whether the destination is locally attached
                               storage or a network device.
                 URL           In the case of a network destination, the URL
                               used for Time Machine configuration.
                 Mount Point   If the volume is currently mounted, the path in
                               the file system at which it was mounted.
                 ID            The unique identifier for the destination.

             When more than one destination is configured, the most recent
             backup destination will be marked with the > indicator.

             When the -X option is provided, output will be printed in XML
             property list format.

     removedestination identifier
             Remove the destination with the specified unique identifier from
             the Time Machine configuration. Requires root privileges.

             To obtain the unique identifier for a destination, see
             destinationinfo.

A. The error:

Error: -69521: Your APFS Container resize request is below the APFS-system-imposed minimal container size (perhaps caused by APFS Snapshot usage by Time Machine)

B. Your response:

I have removed all local time machine snapshots but I am still getting the same output.

C. The documentation:

The word "snapshot", in most contexts, is a generic term and is not to be confused with a "local Time Machine snapshot".

Capisce?

Yes, you have deleted all your local snapshots. But you also have another backup drive. In the example you linked, there was no such drive.

You need to remove all Time Machine backups stores, disable all destinations, etc. You can reassociate your backups later. But following this, you should be fine; worst case, decrypt your drive too (APFS encryption is complicated and has a lot of overhead due to the drive-key/user-key system).