None of the above. My answer is much more mundane.

Krebs almost certainly has professional liability insurance (if not, that would be pretty dumb at his scale) I would call up my insurance company, tell them i've been sued, send them the documents, and then go back to my day.

I would then proceed to follow their instructions, and not care too much about it, unless i was asked to do things that i wasn't willing to do

You’d first move to dismiss for failure to state a claim, and perhaps counter-sue under anti-SLAPP.

IANAL and I haven't read the filing. I'd assume, based on what I've read thus far (and the fact that it's a pretty standard initial response), that he'll file a motion to dismiss and go from there.

It seems plausible that he could have a decent chance of having such a motion granted -- the bar is generally "in the light most favorable to the plaintiff [Ubiquiti]". Based solely on the commentary I've read, it sounds like the complaint could be deficient.