An external drive, HDD or SSD, is controlled by the Operating System and the Logicboard of the computer, whatever a Mac Mac is, so the fact that the drive is external to the Mac is irrelevant, it’s still the same Hardware driving it.
While it is possible that a “loader” will be developed to enable the installation of an OS on a Mac that does not officially qualify, I don’t think there is currently one for Big Sur. The latest one I have seen is for Catalina.
The OP states that his Mac is not eligible to run Big Sur.
That said, if the Mac cannot run Big Sur, then it cannot run Big Sur no matter where it is installed, external drive or internal drive.
All drives connect to the computer that is not capable of running that version of the OS.
Your link contains good information, BUT, is only valid when using a Mac that is capable of actually running Big Sur.