I could understand the Kemerer merkle call if Rivera waved off the takedown because he thought Kemerer had not sufficiently hooked CS's leg. By, IIRC, Rivera waved off the takedown on the basis that time had expired. The rule book suggests that review "may be used to confirm or reverse any call or non-call made." If so, Rivera could have reviewed to see if the takedown occurred before time expired and if he therefore should have called two during regulation, not whether his (belated) call occurred after time expired.
Interestingly, Article 3 below states that Iowa, as the host school, could have arranged for a third party to conduct video review. Don't want Rivera reviewing his own calls? There's a mechanism to avoid that.
The rules are below. For those who might say that I'm spending too much time on this, two feet of snow fell overnight and I'm stuck in the house with nothing to do.
Art. 3. Third-Party Video Review Official. Hosts may secure a third-party registered official(s) to serve as the sole individual(s) responsible for executing coaches' video review challenges. Third-party video reviewers shall review the questioned call without consulting with the referee on the mat. Upon completion of the review, the reviewer will communicate the outcome with the referee who will announce the outcome per Rule 3.13.10.
Art. 4. Description. Mat-side video review may be used to confirm or reverse any call or non-call made on the mat by the official, except a fall. Coaches may not request a video review challenge for control of mat violation penalties. The mat-side video review process operates under the assumption that the ruling on the mat is correct, and only when there is indisputable video evidence that a ruling was incorrect will a call be changed. Absent that evidence, the original ruling stands.