From all articles I could read on font size accessibility, they all tend to say that normal text size should be above 16px, and some say smaller (but not important) text is permitted, but minimum size 12px or so.
So I think the value to check is rather the minimum size in px, so here 21, and it is OK, as it should.
Because here, there is clearly no accessibility issue with font size (21px - 25px).
I’m glad because it seems Utopya.fi (my fluid reference) takes this seriously
So I read the mentioned article which explains a bit better the accessibility issues.
But still, I can’t find in it an easy or direct way to tell if the clamp formula is accessible or not, like your warning seems to do. It seems more complex than that and doesn’t rely on a simple factor.