Matthew was just trying to repair the damage to James, very likely.

Paul: OT is gone except the parts I like

James: OT is still there even oral parts drifting around it.

A huge difference in how the religion should be practiced.

Now if you were a writer 5 decades later and needed to redeem the image of James, while still showing that he was wrong, this could be a good way to do it. It wasn't that James was super wrong, he just misunderstood something Jesus said at one point. Could happen to anyone.