With no context, humanism sounds like it's just something pertaining to humans. Also, at least to my perception, the word has a built-in positive valence. But, it has a much more specific meaning: https://en.m.wikipedia.org/wiki/Humanism
ah, but here "rationalist" isn't being used in that particular way. One of the reasons it is sometimes put in quotes, I think, is to distinguish it from the meaning that you've linked.
A rationalist in that sense is someone who holds the philosophical positions described in that article.
A rationalist-in-this-other-sense is someone who, uh, generally has beliefs in some other collection of philosophical positions, and is involved in a certain community/social-circle .
It is an unfortunate overloading of a term.
Some have given a definition of rationalist (or rationalist-adjacent) as : Eliezer Yudkowsky is a rationalist, and anyone who spends a lot of time arguing with rationalists is a rationalist.
This is quite a different thing that the sense of the word described in the Wikipedia article.
Personally, I'm rather fond of the group, but there are still cases where I find myself using quote marks when describing it.
There's a similar dynamic with rationalism: https://en.m.wikipedia.org/wiki/Rationalism