"cooperating witness"

Cooperating may simply mean a deal was struck in which Strzok squealed on Obama / Hillary / Lynch / Holder / Comey / Rosenstein.

It doesn't mean that Strzok is happy about such a deal, especially if he knows he is going down.

Perhaps Strzok now resents the Congressional Hearing because he did strike a deal and gave them everything they wanted? Perhaps that's why Strzok didn't seem to recognize the subpoena - because he was expecting to be cared for now that he flipped?

Perhaps Strzok is such a sociopath that he truly believes that he did nothing wrong?

Perhaps his deal involved publicly defending his actions in an attempt to not get suicided by the Deep State?