Maybe he's not, but there's little incentive to lie about your age in swimming, and there's a history of crazy high achievement by teens — Ledecky, Franklin, Phelps, etc. I can't find anything suggesting Popovici isn't what he says. Apparently his parents started him swimming at 4 to correct a scoliosis issue.

Relative physiques:

Popovici:



Versus Caleb Dressel: