He's not really saying that frustrated men should go out and get married, so it's not really to the point.
It's interesting to look at this discussion because people seem to get very mixed up between talking about individuals and social structures, and they tend to conflate when he is addressing one or the other, and sometimes then even get mad that they think he doesn't talk about one or the other.
The place where he talked about institutionalized monogamy was an interview, and they were asking him how e (ie society) guards against that kind of situation where groups of men are being problematic in that way. What he said was that one way of addressing it is having a social structure where most people are likely to marry, and that historically that's why that type of society develops, as opposed to the ones where men had multiple wives.
He didn't suggested it's the only thing, but it was an interview question and the interviewer was surprised enough by the answer to stick with talking about it.
That certainly speaks to a certain interpretation of incels as being not just people that were always around, but a kind of group that has developed in response to current social conditions. You could argue that's not so, but it's certainly not obvious that he's wrong, and it's not a sexist theory even if it is wrong.
There are also lots of interviews where he talks about what individuals should do in various situations, and his advice tends to be be very personal and based on his clinical experience. If you are a man who is wallowing in your basement thinking that women owe you something, you should clean yourself up and get a job and find some meaningful activities to participate in. If you are a woman who wants a promotion, you should strategize to get it. Etc.
Clearly he does believe social structures are one way to shape people and behaviour and he often argues about social structures he thinks are effective or dangerous. But he also thinks individuals have a role. I've heard him say in interviews that he ultimately thinks the latter is more important in terms of making change, because you can't change society without changing yourself first - he also said that might be his bias as a psychologist.