Their point is about battery life, and that a thicker phone is the compromise they're willing to make. They don't want a thicker phone just for the heck of it.
I think they are talking about a battery phone case somewhat like the Alpatronix Battery Case (if you want to look around - I just found one at random.)
That's probably going to provide a lot less battery than just increasing the phone thickness, wouldn't it? There's a lot of redundant layers of encapsulation in that approach.
I actually understand his comment to be primarily about wanting a thicker phone such that there is no camera bump. The possibility of a bigger battery is then a secondary benefit.
Either way, my point is that with a case the pixel 9 is mostly flat, and has a battery life that I can't complain about. How big the battery is physically is unimportant to me. Logically it's big enough.