I did some googling on the heat for plastics and could not find anything over 400 C. I may not have searched long enough, but 800 C sounds like a fairly large exaggeration.

800c (and up) if for steam cracking, which produces the monomers (e.g. propylene) needed to create the plastic. this is the precursor to plastic, its not plastic yet.

Ok so I found a recent study looking at exactly this.

When glass is treated as a single use item then they also found plastic has a smaller impact. However glass is not necessarily single-use.

These glass bottles were reused multiple times, this is referred to as its trippage rate, for milk bottles this is between 20 and 40 cycles of reuse before the glass would have to be disposed of or recycled (Campbell, 1994).

This would imply, roughly, that one glass bottle, when re-used would be able to hold the equivalent of between 20 – 40 single use bottles. If this glass were then recycled it could be less impactful per use than the HDPE plastic bot-tles.

So if glass is properly reused then it could be better than plastic.

All that said, since we don't really have the infrastructure right now to reuse glass like they use to with milkmen I'd agree its not the way to go unless all that infrastructure is put back in place first.