People can fool themselves into believing whatever they want. Especially those with certain anti-player tendencies.
Fox may not be an all-NBA defender, and like a lot of players might not be as consistent on that end as one would hope, but he is certainly a superior on-ball defender than Monk. And his length, quickness, and speed allow him to do things defensively than Monk cannot do nearly as well, if at all.
FWIW, #77 of the Mavericks is a poor defender (as are many other offensive stars), but I'll bet you and many others here would still love to have him on this team. So there's that.