It's not like Kirk had all this time to piece it together. The Gorn as a upright starship commander might not fit his expectations.
This phrasing could be handwaved due to, in-universe, the TOS Gorn also not being anything like SNW Gorn and Kirk recognizing that.
Exactly. For all we know it's the first time he's ever seen an adult Gorn.
Or perhaps there's more then one type of Gorn. Use Dogs for example...

On the left, my cute little purebred Beagle named Bruce. On the right, a giant ass Irish Wolf Hound!
They're both "Dogs."