This is not true. Big Sur can read Mojave disks. What *may* be true (I have not tested) is that Mojave can’t read Big Sur disks. Changes do happen in APFS revisions but backwards compatibility for read/write (not necessarily OS install but the installer will update APFS revision generally speaking) is maintained in the OS. Though you of course can’t necessarily have forward compatibility in an OS that didn’t know what the future would be.
Though I believe Mojave will read and write Big Sur APFS though again, I haven’t tested so I can’t guarantee anything there.
As for networking, well depending on how you do that it won’t even really need to be able to read the disk’s file format anyway. The local machine just needs to be able to do that and be able to send network packets with the data - I would imagine that should work fine