I don’t think it has much bearing — whether it’s bottom-dealing or stacking decks or other angles, the problem is the same. But FWIW, what we have observed is him previewing cards while he shuffles (in an “overhand” way, not riffling) and moving cards to the bottom in such a way that he gains information about the hand.
He often tries to circumvent the shuffle-behind method I switched to when this first was detected (for years the dealer shuffled his own deck) by passing the deck directly to the next dealer rather than to the cutter. But even when it is cut, he still has the advantage of knowing what cards are more or less likely to come into play.
So it is not a “cold decking” situation, but gaining an edge through knowledge of card addition/subtraction from the deal. For a very smart player like him, this can be really huge. And, like Postle, his astonishingly good results reflect it.