I've watched that guy's video. The Apple arm64 Macbook Pro doesn't even charge without a functional SSD. I suspect it also doesn't boot off anything else if its main SSD is dead.
Was that an Intel iMac? On the Apple Silicon machines, the internal SSD also contains all the stuff that would be on the firmware flash chip and NVRAM on an Intel machine, so it's required even when you boot from an external drive
Is that new? I ran an iMac with a dead internal drive off of an external Thunderbolt drive for several years.