That seems like the most plausible reason.
Tiling over old tiles isn't that uncommon, and here I suspect they used very little glue (or poor quality glue) so as to not increase the thickness of the floor too much (to avoid issues with thresholds etc.). Thin and/or low quality tiles could be another explanation, as to why they crack lack this..
Either way, I can't think of a way to fix this without ripping up the old tiles and laying new ones.
It's pretty wild, really, that it was too polluted to swim in for a hundred years, and all we humans seemingly did was shrug and go that's just the way things are - can't be helped.