I think his first point is actually quite good, as Tahm, you shouldn't use the stun to eat the enemy, that lacks counterplay since the stun is so easy to land, you should instead use the slow (it's more than strong enough) to apply the 2nd stack and the hit and devour, that would have soem counterplay since the devour is a point and click skill. The problem with Tahm is that his kit is just an unstoppable chain, unliky other champs with similiarly empowered skills, he doesn't need to choose between which one he wants to use, he can just use both of them. If he would have to get either the stun or the devour, he would be way more balanced and fair.
That said, you are correct too in that during the stun, you won't be able to apply again the 3 stacks before the enemy can escape, you usually will have the time to hit him twice then he would bea able to get out of range if he's good and, while later in the game you would have q off cd or maybe be able to stack him thanks to the bonus AS, early game it might be a bit frustrating to see an enemy escape with 3 stacks, but that would still be unable to justify the ability to chain stun + devour, since that doesn't have any counterplay and qss isn't an acceptable counterplay by itself (like for any champion that's countered by it).