This very much feels like an arguing for the sake of arguing type response to me. Given that, what I am typing isn't obscure knowledge in the slightest. Anyway, assuming you are honestly just curious. Sans serif has shown to be the better readable font type on displays. Granted, on modern displays with higher density pixels that is less important.
Either way, both are a better choice compared to a monospaced font.
I'm just unconvinced that UI experts know what good user interface is given the utter monstrosities UI experts create, or advise others to create.
The whole field is trend-driven, to the point good advice becomes bad and vice-versa on a cycle. For example, voice activation is now trendy, despite being known as a horrible UI and not accessible besides; it struggles with accent, dialect, and speech dysfluency, but it's in fashion, so it must be a good interface, right? Previous gurus, such as Jef Raskin, are ignored, and regressions (flat UI) are held up as progress.
> I'm just unconvinced that UI experts know what good user interface is given the utter monstrosities UI experts create, or advise others to create
Sure, I agree with you there. However, text and more broadly readability are not purely UI. Readable text should be part of a good UI, but it is a field in itself and which actually has been quite extensively researched. Unfortunately, as you aptly point out, a lot of people ignore this sort of thing in favor of what is trendy or (in the case of this blog) specific aesthetics.
There are a few things that are quite good understood about what makes text readable on a display. These have been reaffirmed by research spanning decades at this point. Yet they are often ignored in favor of other things.
Either way, both are a better choice compared to a monospaced font.