Thank you! I totally understand this now. So I can also understand why people insist that, for example, Sulcata tortoises should be kept on sand, being "desert" tortoises!? So the case for Egyptian tortoises to be kept on oyster shell in either crushed or "rocks" form, is based on flawed "tradition"!?
I don't have experience with them so I can't say that it's flawed, but it does seem to be following the same path as other species. (no humidity, humid hide, humidity is ok/good as long as substrate isn't soaking wet on top)
soil type substrates seem to be more common in Europe instead of the oyster shell.