There is a single "list" (in whichever storage form, even distributed across several files) of possible models that setxbmap understands. If the argument to setxbmap -model is not in that list, the command setxbmap would not understand it.
My guess (or expectation) is that the man page for xkeyboard-config should produce its output from the same "list" (i.e., the same sources) as used by setxbmap
(at least when using a default configuration; I would understand if man did not know about user added configuration files).
That doesn't seem to be the case.
Command man -P cat xkeyboard-config does not list abnt, nor it is listed here, e.g.
But setxkbmap -model abnt ... works.
In this sense, man xkeyboard-config would be misleading.
How can the "mismatch" be explained?
An additional question appeared after tinkering.
If I execute setxkbmap -model khassss ... (a randomly invented model), setxkbmap does not complain, and even setxkbmap -query prints ... model: khassss ....
What would setxkbmap actually take for the model in this case, to define its behaviour?
Notes:
man -P cat xkeyboard-configdoesn't even giveabntorabnt2as a layout or variant, but I guess this is irrelevant for the question.I am not sure the version I have plays any role in the mismatch referred to (I expect it doesn't)
$ apt-cache policy xkb-data xkb-data: Installed: 2.29-2 Candidate: 2.29-2 Version table: *** 2.29-2 500 500 http://ar.archive.ubuntu.com/ubuntu focal/main amd64 Packages 500 http://ar.archive.ubuntu.com/ubuntu focal/main i386 Packages 100 /var/lib/dpkg/status