If you seal wood you have to seal it 100% on every surface or the moisture that gets in can't get out. My 77 does have some sort of resin on the outside but open on the inside. Not sure if the resin is original or something added after market.
Treated wood is mostly treated with chemicals to prevent bugs and fungus type rot. Trapped or constant moisture from leaks
seems to be the most common wood floor problem. Chemical treatment won't do much to prevent that.
I think the quality of treated plywood in terms of voids and knots might also be something that might drive the choices in sub floor plywood. Treated plywood may have a high moisture content when purchased from chemical injection under pressure. Marine or exterior grade won't.