Looks like a hold as he started to cut.
https://larrybrownsports.com/football/rams-controversial-holding-call-super-bowl-win-bengals/591945
If you watch it in real time, that faint, momentary jersey grab barely lasts a split second and really doesn't do anything to benefit the defender. The hand on the chest before that also didn't disrupt the timing at all and didn't appear to slow down the receiver's motion.
Also, even if you grant that a cut was somehow impeded there, it wouldn't have mattered - the ball was headed right toward the receiver, the defender just took a better route toward the ball. If the receiver had been moving back towards the line of scrimmage like a comeback route, then maybe you could say the defender getting a quick grab/push would have mattered, but given where the receiver was running, the defender would have been in between the pass and the receiver no matter what.
Pretty bad call, given the spirit of the rule. Really bad call given the circumstances of the play/game.