I think they're claiming that the popular vote has no real relationship to political legitimacy in the US, which is true, because the electoral college/FPTP sys

I think they're claiming that the popular vote has no real relationship to political legitimacy in the US, which is true, because the electoral college/FPTP system is explicitly designed to render the popular vote essentially meaningless through layers of abstraction and gatekeeping. It's only really useful for propaganda, and not even that useful - Republicans claimed a sweeping mandate from the masses even in 2016 when millions more people voted for Hillary Clinton.