Apple is putting raw NAND chips on the board (and yes soldering them) and the controller for the SSD is part of the M-series chip. Yes, apple could use NVMe here if you ignore the physical constraints and ignore fact that it wouldn't be quite as fast and ignore the fact that it would increase their BOM cost.
I'm not saying Apple is definitively correct here, but, it's good to have choice and Apple is the only company with this kind of deeply integrated design. If you want a fully modular laptop, go buy a framework (they are great too!) and if you want something semi-modular, go buy a ThinkPad (also great!).