It was ages ago that I read online that Clinton graduated Level 3 of PNH; it might've even been on a horse forum. I don't care to call them & ask, & it's highly plausible that Clinton'd leave his time spent in PNH out of his bio (if that's where you got the above), so we're left with whoever wants to, to call him & ask. (Although there exists the possibility of his denying it, competitiveness could prompt this. If he denies it, one could call PNH & see if they seem to be upfront about it.)
I don't see, however, why the possibility of Clinton's having studied PNH is met with such incredulity, here. If I said he studied under Jane Savoie, ok. lol!