Many argue that rationalism itself, the elevation of reason above all other values and forms of knowledge, descends directly from Plato. I have only read a few of the Socratic dialogues myself, but I certainly have gotten the impression that this is a reasonable take on Plato and a departure from earlier traditions such as the Pythagoreans and other pre-Socratics -- most notably Parmenides -- who did not take a purely rationalistic view.
The person I'm replying to isn't talking about "rationalists" in the sense of Descartes and Spinoza, they mean the people in the Bay Area who read Eliezer Yudkowsky and talk about AI safety. And those people are not Platonists in the least. If anything, the usual accusation levied at them is that they drift too far in the nominalist direction. This is evident in pretty much all their writing, so I'm wondering where the GP is getting the impression that Platonism is their intellectual original sin.