I'm not familiar with a distinction between a more common usage and a more jargony usage of those two words. Is there one? Actually, I don't think I really understand any usage of the word "humanist" very well.
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.
