??? what are you disputing here? IPRs, or defending against an infringement suit, or what?

Doesn't look like a dispute? I think they were trying to supply you with a phrase that slipped your mind.

the question is the antecedent basis of "it"

You don't need to get an IPR, you just make an invalidity counterclaim. IPRs may or may not be desirable for a number of reasons.

TIL. We're talking about US law, right?

I've never heard the term "invalidity counterclaim" and I was in Google Patent Litigation where suits come in constantly. IPRs were pretty standard. Maybe the lawyers did use it and I just never heard it.

If something was going to trial, then "invalidity" was one of the possible defenses, as was "non-infringement."

Who are you making this claim TO? The court? The PTO?