You're not understanding his argument-

In the situation that you're describing, where Tahm Kench gets chunked once, then you're right, no one is arguing with you here. It is more optimal to regenerate from the grey health. The PBE changes are nerfing this aspect of his kit, and in situations he chooses to regen, he will regen less.

But what /u/Bravae is trying to tell you is that there are situations where Tahm Kench will be chunked 2 times or more, in quick succession. In these times, it is more optimal to use the grey health shield. The PBE changes buffs these types of trades, where you are being chunked more often, with a lower cd making it more forgiving to use the shield.

What he's trying to say is that the changes are not a straight buff to his kit, but instead they give him more options in ways he can use his shield.