I'm happy to revise my node so it explains things better.
You discuss "what is the prototype of a function after parsing" (that's knowable contrary to your claims) rather than "what will be the prototype of a function after parsing" (that's not knowable).
Or put differently, you talk about one instance of the parser when the problem is with differences in the output of different instances of the parser.
If there's a fallacy, it's assuming that the output of an instance of the parser will be the same as the output of another instance of the parser. That would be a very bad fallacy to make since that's the general hypothesis that needs proving or disproving.