Reading about this, you're right definition-wise (Glass is not biodegradable), but in practice Glass is degradable to safe materials - and this happens naturally - which is what we really want.
polypropylene is indeed pretty inert when it's not photodegrading (though there are some interesting studies finding biodegradation https://scholar.google.com/scholar?q=related:yE8AZEyCArgJ:sc... but all in aerobic environments afaik), but when it does biodegrade or photodegrade, the degradation products are pretty harmless, as with glass and sand
it does photodegrade reasonably fast if exposed to sunlight, in years rather than decades (or millennia as in glass)
implanted soda-lime glass causes irritation in a similar way, by the way; i have a granuloma in my foot right now which is encapsulating a splinter of glass i'm biodegrading. the issue with polypropylene is that it's, believe it or not, more resistant to biodegradation than glass, at least inside the human body
the issue with polychlorinated dibenzo-p-dioxins is not just their persistence; as pointed out above, things like glass, sand, and polypropylene are even more persistent. the issue is that in addition to being persistent, they're toxic—but not acutely so, except in extreme yushchenko-like cases
I'll note though that biodegradability is not the only consideration, and there are other important advantages to plastic straws.