They made a pro-longtermist video in association with open philanthropy a few years back, The Last Human or something like that, the summary was pretty open about the connection.
I don't think the shadiness is specific to rationalism, see also that bizarre KG video claiming it's scientifically impossible to lose weight by exercising that coincided with the height of Ozempic's hype.
edit: The Last Human came out at 2022, the same year the McAskill book arguing longtermism was published, what a coinkidink.