And I can and do use the shorthand, Antifa, without asserting there exists some organization with that name.

Perhaps.

But that distinction is rather the point in many of these conversations. Joe Biden is frequently mocked for having noted that Antifa was "an idea," not an organization in the 2020 presidential debates - as with the Turley quote that moved the thread in this direction. But as you seem to recognize, that position is correct.

And it's important - because Biden was raising that point to illustrate how ignorant DJT was about Antifa during that debate. Trump at that time was claiming that he would have Antifa designated as a terrorist group, which was just nonsensical (because, again, Antifa isn't an organization). Trump genuinely didn't understand that distinction, and didn't understand that his own FBI had reached that same conclusion.

yahoo.com - Biden says antifa idea not

So whether your use of Antifa as a shorthand doesn't imply asserting of the existence of an organization, other conservatives (like Turley) will sometimes use Biden's correct insistence that Antifa is not an organization as a criticism of the President. And it's appropriate to point out that criticism is wrong.