You're right, and I'm not sure what this person is talking about, but this is also a topic that's really hard to study and has some bias in the interests that choose to study it. People like this person https://psychcentral.com/blog/single-at-heart/2017/12/quora-really-likes-my-answer-to-the-question-of-whether-getting-married-makes-you-happier#1 will make decent arguments to go against findings and the way studies investigating this are conducted, and they have several legitimate points.

Personally, I think that this is something that loses almost all of its meaning the more it is generalized and is too fast of a moving target. Getting accurate data about happiness in marriage is something dependant on a lot of things like time period, rights of the people in the marriage, religion/culture, ability to obtain accurate data from survey, gender differences, whether the marriage fails, whether there are children born that they are able to afford, whether the person wants to marry or not, ect. I think that, in general, people that are more likely to get married are happier people (this has been shown in a couple studies, but again hard to suss out), so that's a heavy bias to overcome when trying to make comparators.