15 minutes is just not very long at all when cold brewing even at room temperature. And this was refrigerated (4C) so I'm sure the flavor would be minimal. You would normally cold brew for at least 8 hours at that temperature. So much less of the tea is ending up in the tested water.

If you look at the pictures of the plates the one with cold brew and where the full tea bag is added to the plate seems to have a good bit more growth than the others (there is a larger image of this one). So it doesn't seem like the spores are necessarily unaffected by the boiling just that some survive (or maybe they all survive but only some decide to start growing?). The thing I find strange is that the tea bag that was directly added without any brewing doesn't seem to have much growth at all. Actually, also that in the one with the tea bag added after cold brewing that has extra robust growth much of that growth seems to happen away from the visible pieces of tea.

I agree that the author has great style and I find it interesting to think about where and how bacteria (and fungi) like to grow.

I think it’s just that the bacteria in the hotter container woke up to a warmer, nicer environment. Warm temperatures are much better for bacterial growth. While it was too hot they didn’t seem to care at all since their spores protect them completely from damage.

Agreed - I really enjoyed the writing. I'd have normally skimmed for the conclusion but ended up reading it all. Shame that that's so rare these days.

They could have some coating that make them stick to the content of the tea bag, basically affecting their solubility.

Perhaps just random variation? They only did one plate, after all.