The SPLC is describing anti-government groups among the anti-mask movement, not anti-maskers as a whole, but the Venn diagram of "anti-maskers", "right-wing militias", "conspiracy theorists" and "white supremacists" does overlap. Not perfectly, but also not inconsequentially.

The phenomenon you described as having happened within the Tea Party has happened everywhere among the right end of the American political spectrum over the last decade. Rhetoric which was once confined to the lunatic fringe of talk radio and forums is now mainstream. QAnon emerged from the bowels of 4chan, exploded and became popular enough to win elections, and now the Republican Party enthusiastically stands behind a Congresswoman who believes in Pizzagate and Jewish space lasers. The former president of the United States retweeting nazi memes was not even a story, it was just another Tuesday.

And yes, this has happened somewhat among the left, but not nearly to the same degree, and mostly as a reaction to what's happened on the right.

But can you really blame the left for seeing nazis everywhere when nazis keep showing up everywhere?