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These instructions do not make the microSD bootable. It’s always bootable by default with the first boot priority. Pinephone always automatically tries to boot from it. If the boot fails, only then the eMMC will be tried. This is a very big distinctions between the two phones. In case eMMC dies, Pinephone will stays almost as useful as in the beginning, not so for Librem 5 (it’s almost like a planned obsolescence for th elatter, but it’s for technical reasons actually AFAIK).
Upd. See also: https://linux-sunxi.org/BROM#A64, https://linux-sunxi.org/Pine64#Boot_sequence.