Hello, do you have pictures of the under side of the back area?
Typically the builders bond this part directly to the transom because this is where the outboard mounts/clamps to the transom and it needs to be solid and not allow any give. It looks free from your pictures, but the vertical wall behind the lip is probably fiberglassed or glued to the transom. When I removed this section from my boat it had some super funky glue pseudo permanently attaching this to the transom. I wasmodifying my cap so I just cut it into pieces.
If you are trying to get the cap off in one piece with the intention of putting it back, you still may need to cut it where you know its free but plan to fiberglass this section back in when putting it back on.