Probably contains some particles from the grindstone yes, but I wouldn't think in anywhere enough quantity to have a particularly deleterious effect on people's teeth
Depends on the type of grinder. A saddle-type quern absolutely puts enough rock into the flour to degrade teeth, as seen on ancient egyptian remains.
When I was a child my parents did a brief experiment grinding our own flour with a cheap grindstone. I can clearly remember the sensation of the bits of stone in the bread.
It is possible to make flour without this problem? Almost certainly! But this n=1 anecdote demonstrates it’s a thing that can happen.
I eat stone ground flour (in bread form) almost daily and no dentist has ever registered a complaint.
I think GP meant "Primitive stone ground flour" given context of GGP.
More than toothpaste?
Toothpaste usually uses microplastic or ground walnut shells to prevent exactly this issue.