The word "rationalist" is thrown around here a lot in various contexts. There have been complaints about this before, though I am not quite sure how to locate them quickly (obviously searching the site for "rationalist" turns up quite a few results.) Here is an old discussion.

Here are a couple:

• A "rationalist" could be someone who regularly reads and/or participates on this site. A clearer term might be "LessWronger." This category of people is often intelligent, socially awkward, well educated, and has a particular techno-liberaltarian slant. Things vary but these are good guesses.
• A "rationalist" could be someone who believes in the virtues of rationality and wishes to improve themselves. This seems like an appropriate noun form of rationalist to me, although "aspiring rationalist" is a nice term I've heard.
• A "rationalist" could be someone with a specific goal, which they are concerned with pursuing as effectively as possible. A clearer term might be "rational actor."
• "Rationalist" is often used as an adjective describing arts, such as in "rationalist!fanfiction." In this context I usually interpret it to mean arts which glorify the virtues of rationality, by having heroes succeed and be awesome via planning and cross-domain thinking.

As for the statement "unencumbered by education," there are two possible interpretations I have: First, the man who started the site, Eliezer Yudkowsky, is an autodidact. References to knowledge without schooling may refer indirectly to him. Second, there is a significant and hopefully healthy amount of disrespect for authority on this site; including for academia and modern schools.

I am about 80% confident that someone has already composed this sort of a list, and that theirs is better than mine.