The author of that book was not an eyewitness. If there was a historical Matthew he would have been an illiterate Aramaic speaking Jew not a highly literate Greek speaker. Even if the historical Matthew managed basic literacy he definitely would not have been witness to all the events that happened in secret, such as Mary speaking to the Holy Ghost alone.

And Mark could have copied from Matthew instead.

No. Matthew is a superset of Mark and religious writing get longer with time not shorter. 85% of Matthew is from Mark. Additionally we see Matthew break chiastic structures that Mark had to get results he wanted. Finally historically it doesn't make sense. Paul was either freshly dead or very old so it would make sense for Mark to recast Jesus as Paul, by the time Matthew was written the Jerusalem community has been mostly destroyed so they needed recruits from Jews. Hence the echoing and rewriting to cast Jesus as the new Moses.

This “genetic fallacy” or whatever you made up is purely ignorant

Logical fallacies. I didn't identify them first. They were discovered 5 centuries before Paul made up the Jesus myth.

as people who actually knew this stuff and where it came from are the strongest and most reliable sources.

Argument from authority. Present your evidence. The attributions of the Gospels were done by people over a century later. If it is so obvious why not just present your evidence?

Saying “We know Mark was written first because Matthew copied from Mark” doesn’t make any sense - as Mark could just have easily copied from Matthew. And I’m the one you say doesn’t know logic?

Repeating yourself.