Nobody said it was all centrists, but there’s no such thing as benevolent neutrality when fascists are seizing power. There’s just not.
There’s also no leftist extremism in sufficient numbers to be a threat in the US. In fact, the vast majority of ‘the left’ in the US are actually traditional conservatives.
My anecdote may not be statistically relevant, but if you’ve spent time online in the last seven years, you’ll have encountered similar situations on a near daily basis. Pretty much every city has recently played host to openly Nazi rallies, and that’s because they’re comfortable coming out from under their rocks since one of the major parties accepts and supports them.
Currently in the US, the split is roughly 30% left of centre, 30% right, and 30% either centrist or not paying attention. That’s exactly the same breakdown of society as when the Nazis seized control in Germany. Centrism in the face of fascism helps the fascists succeed. We know this, and it’s part of why historians have been raising alarms.