MacOS catalina install error – this disk doesn’t use GUID partition scheme (Au Contraire!)

I am trying to upgrade an Late 2013 iMac from OS X 10.9.5 to macOS Catalina.

When I run the catalina installer, it hangs a bit and eventually let's me try to click the drive to install on. After clicking furiously around on it, it eventually pops up the error about I need to use the GUID partition map.

Checking diskutil, I'm greeted by this:

$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            999.3 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3

Ok, confused. Let me just check disk utility:

Also says GUID partition map. But what is that weird unlabeled partition above "Macintosh HD". Reboot into recovery mode and run disk utility. I see it's "OSX Base System" or something along those lines. Now when I go look at the partition map on THAT disk? Partition? It says "Apple Partiton map" or something like that. I can't change it in the recovery mode. So what's up? Why does disk utility when running the system not show the "real" partition map type? We can clearly see Macintosh HD here shows GUID partition map. Very confused.

I'd like to upgrade this without reformatting/erasing the drive. Hoping there is some doc or oddity I have not discovered to fix this.

Thank you,