So, first, Hector is an expert (they're project lead of Asahi Linux, so there are few people outside Apple on the planet who know as much about how ARM Macs boot as they do), and their thread is worth reading.
That said, I disagree with their SSD/flash distinction. An "SSD" is really just a bunch of flash chips with a controller that coordinates them (including allocating them smartly, etc.). In Apple's case, that controller is on the M1 itself, so the module you insert is really just the flash chips, and isn't a full SSD of its own. But the end result is an "SSD" nonetheless.