I had a biology teacher who was fond of repeating " everything is toxic, it's just a matter of dosage"

And it's true, you can die from water toxicity.

Water and air are ubiquitous, so we've naturally evolved defenses against overdosing.

Sugar has always been rare and valuable as a food source. In fact most organisms existence revolves around collecting enough chemical energy. It's only in the last 100 years that sugar has become cheap enough for many humans that overdosing is now a problem, and we have little in the way of evolved defenses that keep us from overconsumption.

Sugar addiction is real.

This is in the same category of statements as "ACTUALLY, everything in food is a chemical".