Do all the non politically affiliated people who hate billionaires not count? Or why is the granularity here important? Your point is stronger the other way!

Populism is a "thin" political ideology that often gets layered on top of other political ideologies, both left- and right-wing. It simply means "policies that appeal to ordinary people" (vs. a rich and perceived corrupt elite). By definition, someone who hates billionaires simply because they are billionaires is a populist. They might hate other populists that have attached themselves to other political ideologies (and have different scapegoats or preferred policy prescriptions to rectify the inequality), but they are still a populist.

> By definition, someone who hates billionaires simply because they are billionaires is a populist.

You've ascribed to them an ideology they don't hold. They don't hold that view "simply because they are billionaires".

Meaning is derived from real usage, not from dictionaries. Descriptivism has won. And in the real world, it's simply used as a cheap shot to claim that certain policies or thoughts are only for the winning of votes rather than well thought out or other "ideology" based.