The difference between MN and the other site is that MN has a report system and set of rules and guidelines, and they explicitly ban racism. Is their system perfect? No, but at least they have one. The other site didn't have any rules and barely had any moderation.
There's also something very distasteful about setting up a forum to troll another forum you've been banned from. The PBP creating new accounts here in order to spam and promote their new forum (both by starting threads promoting themselves, and by directly DMing people going "check out my new forum, it's the new Mumsnet but better") crossed a line.
If they wanted to be rid of MN and set up a new forum, why not just go off and do that? Why the fixation on trying to steal posters from MN, monitoring what's said on MN, trying to control what happens on MN, interfering in MN moderation?
It's like a husband who gets a new girlfriend after his wife kicks him to the curb, talks about how his new girlfriend is so much better and hotter than his ex, but still spends every second stalking his ex-wife.