Not sure what replacable has to do with thickness.

When I bought my first smartpone, a Moto G (1st gen) it was as flat as any phone most people carried around at the time (2014, I think). And the battery was replaceable.

I think also Samsung phones had replaceable batteries then. And this was the case for a few years after. Until it wasn't.

Devices didn't suddenly get thin when batteries were glued in. Why would they?