"if all files need to be treated anyway, why does the order matter?"

The sort is to ensure that the same list is produced whatever the OS/git version is used.

Previously, POTFILES were sometimes recreated with a different content order, even if there was no changes in the source files. That implied regeneration of .pot files, and so possible modifications of git tracked files.

mawk is lacking two features used in the update-potfiles.
I can get around one of them (match()), but I do not really want to implement a sort() in mawk.

So, the question is : what prevent gawk to be used on Debian ?

We could possibly replace the awk script, but only if that's the only solution.