Those aren’t M.2 slots, and they’re incompatible with NVMe drives. They take Apple’s proprietary NAND memory modules and connect directly to the memory controller on the T2 chip, whereas NVMe drives have the SSD controller on the module itself. The second slot is for capacities greater than 256GB, which operate in an interleaved fashion for faster speeds.
Ah you’re right. The pin layout is indeed different, but at first glance it looked a lot like an M.2 slot. Interesting design decision... I get why they separated the T2 chip and controller from the flash storage, but the new interface is curious. It’s like a lightning connector for your SSD lol