Since when is gluten "toxic", not just "leads to an allergic reaction in a minority of the population"?

There is far more in wheat than gluten. Seeds of many kinds are filled with specific chemical compounds that are toxic to mammals. Plants actively participate in chemical warfare at many levels, via the roots, leaves and seeds. https://en.wikipedia.org/wiki/Seed_predation#Seeds_and_their...

How dare you let reason get in the way of the pitchfork masses.

"Reason"? I think you spelled "assumption" wrong.

But fermentation (like in sourdough) can fix this right?

Yes, fermentation lowers not only the gluten it also reduces the toxic chemical defenses built into seeds as well in many cases. Beware though, most "sourdough" available in anything less than an artisan bakery is generally standard white dough with lactic acid added to it and some weasel words on the label to make it sound like it's naturally leavened.