The backstory to this question.
After erasing crypto_LUKS headers as specified in frostschutz's answer, I tried following this guide again. I got stuck at mounting Mac's EFI partition to /mnt/boot again, but now with a different error, namely wrong fs type, bad option, bad superblock (see 1st screenshot here). Also, while lsblk showed my partition as crypto_LUKS before erasing the headers, now it shows nothing about it (see 2nd screenshot here). I tried to check it with other tools, and cgdisk, on contrary to lsblk, shows the UUID of the EFI partition (see 3rd screenshot here). Also, fsck doesn't show anything about /dev/sda, it says something about MBR (see 5th screenshot here). Could it be the problem with MBR/GPT tables?
Mac's diskutil shows the same info about EFI partition as in the previous question (2nd screenshot here).