There are two pretty simple reasons why Apple Silicon Macs don't have user-upgradeable storage yet.
The SEP (Secure Enclave Processor) inside the Apple Silicon chip encrypts the internal storage in hardware. When you swap in a new SSD, the SEP doesn't know how to decrypt it because it doesn't have the keys to the new disk, so the Mac cannot boot.
In addition, iBoot (the Apple Silicon bootloader, inherited from iPhone) is only capable of booting a macOS kernel off of the internal storage, recognizing a Touch ID button press, and putting an Apple logo on the screen. Nothing else. Without a valid filesystem or recovery partition on the startup disk, the Mac cannot boot.
It appears that the only way to get an Apple Silicon Mac to recognize a new SSD is if you perform a DFU restore on it using Apple Configurator on another Mac, which reprovisions the machine and places a valid filesystem on the internal storage. So it makes sense why Apple avoided the trouble of having to explain this and instead decided to say that storage is not user-upgradeable.