that mushrooms are not natural food for Hermann's tortoises
As far as I'm aware they aren't - their diet naturally consists of leafy-greens (lettuce, clover etc) and some vegetables (cauliflower, broccoli, carrots etc) and small amounts of fruit (apples, strawberries etc). Any diet in captivity I would expect to match their "natural" diet for the most part.
Mushrooms naturally grow in European woods
Hermann's torts aren't naturally woodland tortoises in the same way as e.g. Redfoots (who will happily munch on mushrooms by the way!) so while most edible mushrooms probably wouldn't hurt a Hermann's they likely wouldn't even really consider them food. Their "hunting" ground is generally meadows.
and so tortoises had must have have learnt how to enjoy them.
It's not that simple at all. I imagine their habitat encompasses a great many things that they wouldn't enjoy eating.