Are you sure about this? I have no particular knowledge of the topic but my understanding was that there are tests to distinguish between naturally-produced and artificially injected testosterone.
If it was simply a matter of accepting any value below a certain level as fine then by your own logic many athletes would already have been doping up to just below that level, which I haven’t seen any suggestion of. If they were, that would surely have been part of this Caster Semenya conversation.