Surgically altered to be a human and totally undetectable to a full body scan by the old ships doc, which is what he went through at least twice so far when returning from Klingon ships?
If it does turn out that he's Voq then it would surely have to be a mind transfer to Clem Fandango's body, and not a surgical thing which would surely leave traces (klingons) which could be detected. Also the DNA signal scrambler he wore onto the Ship of the Dead would probably get confused.
In a previous episode she said she's take him to her clan but he would have to be prepared to sacrifice everything...so my guess is mind transfer to Clem, creating Ash. The flashbacks are because at the moment Ash's personality is dominant but L'Rell will be biding her time to trigger the resurgence of Voq when opportune.
Or something.....'cos mind transfers by the Klingons doesn't sound like something they'd normally do, unless they subcontracted it to Dathomir......