I think use of the word subhuman was slightly lacking in tact in the context of a public forum. But I won't say I disapprove of it.
The thing is I know it's not meant literally. They're not saying that Biden is literally, in actual DNA terms, an inferior breed, separate from a full-fledged human being. They're just lashing out with it in anger as a hyperbolic pejorative. Finding edgy words to stress just how strongly they disapprove of the guy, at a time when I imagine no language feels sufficient to communicate the feelings going on. A eugenicist or fascist who uses that word means it literally. It's not even an insult to them. It's a statement of fact (banality of evil, indeed).
And I'm the type (growing increasingly rare, it seems) who believes that the actual meaning of what's being expressed is more important than the specific arrangement of letters used to express it. And that actions or serious statements of intent to action are very much more important than both.
So no - I won't say I disapprove of the use of the word subhuman in this context, because even though it's a word associated with usage by bad ideologies, it wasn't being used to express one of those bad ideologies. It wasn't "close to Nazi territory" because it wasn't expressing anything close to Nazi ideas.
In fact Biden, in terms of the actual meaning behind his words, was much closer to Nazi territory when he said that he wouldn't sign M4A. Because that would literally, in the physical world, result in perpetuation of ongoing systemic violence that disproportionately targets based on racial minority and lineage. If using the word "subhuman" in a non-literal way puts someone closer to Nazi territory in your book than ideological dedication to policies that mass cleanse the poor, then I think I don't have anything more to say to you.