I don't see what the big deal is here, so what if Matthew didn't write what we call gMatt.
What if someone had written this down after hearing Matthew preach or attended a range of meetings with Matthew?
The ONLY honest, and academic position to take here is to have the decency to accept that we will never know for sure who wrote any of the Gospels.
I would like to add that I do find it a tiny bit annoying that some people find no problem in stating that this text and that text WAS written by 'X' or 'Y'. This only displays an ignorance of biblical studies and the discipline of history in general.
The VERY VERY best any one can say about any Gospel we have is that it MAY have been written by the Evangelist whose name has been given to it.