It's definitely canon for Dredd. It's referenced several times subsequently in 2000AD issues and elements of it became important plot points later on.
Whether it is canon for Batman is murkier, I remember looking into it and finding some suggestion that it was but it's less clear-cut and Herochat is down now now so I can't find the post I put together with the evidence for it being canon at the moment.
Either way, I think people tend to put too much emphasis on what is hard "canon" when it comes to crossovers. At worst, they give us an idea of how a writer views characters relative to another so even when crossovers aren't in-continuity in a strict interpretation they can be useful evidence IMHO.
I'd back Dredd against Moon Knight with or without the Batman showing mind you (I maintain that the Batman h2h stalemate isn't actually Dredd's best h2h showing, although it is the most easily recognisable to non-2000AD readers of course). I don't think it would be easy by any means though, Moon Knight can give any of the top unpowered guys a run for their money.