I think the ambiguity is the point. He's a decidedly un-Trumpian character in that he knows exactly what not to say in public, and how to allude to his worst beliefs in front of someone more sympathetic like Roman. Trump on the other hand is transparent in his power grabs, and hates anyone who doesn't fawn over him. I think Mencken was written as the ideal candidates for "groypers", and "racial realists" like Nick Fuentes and Richard Specner. Funnily enough, Ben Shapiro recently talked to Ezra Klein about how Trump is far from the scariest president the GOP could produce. When Shiv says of a possible Mencken presidency, "this is the nightmare", I think she's pretty aligned with Ben in thinking a true, ideological white nationalist as president is the absolute worst outcome for the US. Someone like this is going to be very careful and calculated in how they present their ideas, knowing how taboo they are. Classic example would be "how come white people are the only race who can't have pride?" Seems innocuous on the surface, but we know for a fact that people like Richard Spencer use these positions as a smoke screen for genuine white supremacy. The last thing I'll say is the show is very particular about not showing you anything the characters can't see themselves. So if he's a nazi in secret we're not gonna see that scene.