A cake can never depict an upside down cross. There is no 'up' or 'down' on a two dimensional horizontal plane.
But more to the point, I would imagine, seriously, that the baker would have a case against the group and the lawyers for harassment and frivolous lawsuit if they try to pursue any legal action against him.
The whole thing is absurd. And, of course, we all know the Left would never try any of this crazy nonsense with a Muslim bakery.