Antislapp is definitely a procedural immunity that should be subject to interlicotorry appeal, but I kind of wonder if the aren't walking randaza into a butcher shop to make bad case law for himself, something like a blanket ban on attempting to file out of state antislapps.
Anti-SLAPP from other states has definitely been applied. One particularly prominent example was when Stormy Daniels sued Donald Trump in a New York court, the case was removed to federal jurisdiction, and ultimately tried in California federal court. The California federal court used New York's choice of law provisions to decide that Texas anti-SLAPP applied (because Daniels was a Texas domiciliary), and dismissed the case and sanctioned Daniels. (If contemplating this makes your brain hurt it should).
So as much as the typical response to this (even from me) has been "huh what?" it is not in fact completely out of left field.
A major difference, though, is both Texas and California have particularly strong anti-SLAPP laws that are actually pretty similar in their application. I think if California had chosen to use its own anti-SLAPP law the result would have been identical.
My assumption continues to be the appeals court is only going this far because of the novelty of the appeal. If so, that's usually not a great place to be. I 100% could see them doing exactly what you say.
I was actually surprised he even got a foot in the door on this. Although generally if a court calls your argument "novel," you aren't going far. They have not actually used that specific language but I think that is the direction they're coming from.
And as I pointed out, just to decide they don't have jurisdiction means they at least have to consider the merits of the case itself. If there is no right to Colorado's anti-SLAPP in this situation (as I think there is not), then there is also no right to an interlocutory appeal.
But they actually have to address the first issue to get to the second.
Randazza apparently ponied up enough legal reasoning that the appeals court may be skeptical, but still has something to address. As far-fetched as I find the argument, it is not facially frivolous.