Except the thesis is that the mainstream left (which would be the Democratic Party) is already just as extremist as MAGA (which has become the mainstream right.) When people resort to calling Joe Biden a communist, or even Barack Obama, because there's no one in power on the mainstream left to whom the label "far left" could reasonably apply, it's an obvious false equivalence. The "Nazis" are saying Nazi things and doing Nazi things while the "Communists" are at best European style Democratic Socialists but are framed as hardline Stalinists in order to make the "pendulum of extremism" narrative work.

You're correct that the increasing radicalism on the left didn't materialize from a vacuum, it's a result of the populist left being disenfranchised by how not leftist the Democrats are in practice, it isn't a result of iterative radicalism within the Democratic party itself and it wouldn't exist if the Democrats were anywhere near as extreme as the Republicans make them out to be.

If you're looking at both parties and seeing two equal and opposing ideological forces then I think you're deluding yourself. Maybe in a few years that changes if the left can do to the Democrats what MAGA did to Republicans, but at the moment it doesn't reflect reality. What we really have is a far right party and a center right party.