You sound like you're using the word "rationalist" in a way which is at odds with the usual meaning of the term. How do you define it, and why do you define it that way?
Sounds to me like GP is drawing from personal experience consuming a variety of rationalist venues online, and making generalizations from there. Thus, they're applying a descriptivist methodology. I'm not sure that your prescriptivist methodology, working from a definition of rationalism, will produce a satisfactory conclusion. When folks approach an issue with an inherent disagreement about descriptive/prescriptive approaches, the most common outcome seems to be a spiral towards "no true scottsman."
I'm not trying for a prescriptivist approach, merely one based on my own experience with the term "rational" and similar.
The Efficient Market Hypothesis famously assumes that investors are "rational." It's been widely ridiculed as unrealistic, because investors are human, and wont to irrational action. Perhaps one shouldn't assume that rationalists are rational; a more, shall I say, reasoned approach would be to evaluate those espousing Rationalism on the (little-r) rationality of their words and actions. Though, there are those who would be offended by one's findings, yea or nay.
I suspect they’re using it the same way it’s used on Less Wrong[0].
For all their sneering at traditional philosophy, rationalists (and specifically the Less Wrong community) love to rewrite traditional philosophy in techy terms while missing most of the underlying meaning and depth. For examples, large portions of The Sequences are Plato’s The Republic but worse or Wittgenstein’s Philosophical Investigations but worse.