I'm not going to pretend like I know anything about trademark law, but here are two articles that give more detail about what's happening in this case:
One from when Banksy first opened his pop-up shop:
Anonymous street artist Banksy has opened a pop-up shop in London in response to an ongoing trademark dispute with a card manufacturer. The attorney representing the card company reacts to the opening.
www.worldtrademarkreview.com
And one about the ruling from a few days ago:
Street artist Banksy has had a trademark for one of his well-known artworks cancelled for being filed in bad faith, with the panel claiming that Banksy’s Gross Domestic Product shop “undermined” the case.
www.worldtrademarkreview.com
After reading those articles, I admit that I still don't fully understand everything. It does at least seem like I was wrong in thinking that Banksy is trying to make this some kind of performance art thing. It does seem like they're legitimately worried about their trademarks. One thing I was right about is that Banksy apparently has a terrible lawyer.
About the actual ruling, I don't know what to tell you. It seems like it's impossible to have anonymity and a trademark. That seems like it makes sense, as a person who knows nothing about trademark laws, because if the person is anonymous, then how do you even prove that they made the art?
And like I said before, I feel like Banksy would do a lot better for themselves if they just stopped caring about their trademark. Yeah, their art is being exploited by capitalism now, but that was inevitable. If they didn't care about it, then they could at least still appeal to their anti-capitalist ideals. But now they've thrown that out the window by trying to commodify their own artwork. Granted, it sounds like they're only trying to do that for superficial reasons to prevent others from commodifying it, but it still seems like a move that's unbecoming of the actual art itself.