Show me any hardware that is 100% "libre"? Even the pinephone itself has plenty of closed source blobs running as firmware.
If you take some random windows laptop off the shelf, it will boot linux (and continue to do so in the future) because they have to support UEFI. If you take a "linux" friendly vendor off the shelf, you may even have coreboot or something on-board.
But with this apple situation there is no guarantee the next generation of hardware won't lock you out, or they might push out an OTA firmware update that locks you out. It's like porting linux to the playstation or something.