NatPop ideology in Kaiserreich/Redux is kind of a misnomer anyways. Really it would be an improvement if they just removed it and split it into two or three different ideologies, but I guess the subideologies category covers it.
Hence why they justify calling Huey Long a NatPop at the start of a USA run because be is a populist, despite the fact NatPop is colloquially seen as the fascist shoe in for Kaiserreich down to using Vanilla HOI4's color for the ideology. It is extremely subversive in my opinion given like you said, the absolutely massive number of ideological theories that fall under National Populism from revolutionaries to Italian Fascists.
Of course this just follows the usual trend of Leftists ignoring the Leftist roots of Fascism, because that opens up a whole can of worms for them on whether or not someone from the Right can care about worker rights, environmentalism, etc.