Well, if he had said they were exclusive, he shouldn't have been talking to women on OLD at all. I'm not in 2 minds about that. The only thing I am in 2 minds about is whether to end a longer relationship over this if it's going well otherwise.
It doesn't bode well and this kind of dishonesty is never a good sign. I just suppose, if it was idle chit chat, not leading anyone on, not seeking out meetings (obv one could ask 'what's the point?' and 'what would have happened if he'd met someone he found really interesting?') It might not be the same as if he was doing those things.
Does he have an explanation and how have the last 7 months been? Also if he admitted this openly and apologised it would be different to if, say he had tried to chat the OP's friend up and she found out that way.
Suppose I'm also thinking it was lockdown 7 months ago, maybe this was his way of having a bit of company.
Doesn't sound great in all, but there is a chance he might not have meant anything by it. It is really unusual for me to consider condoning messing around elsewhere, i just feel that depending on those factors this could possibly be a grey-ish area rather than unforgivable.