How to make macOS APFS Container show up in the boot menu again?
You could try using gdisk to change the second partition to type AF0A. Note: klanomath's answer lacks the gpt remove ... command due to the nature of the question you have referenced. The type AF0A exists in versions 1.0.4 and 1.0.5 of gdisk. If you have an older version, then you have to enter the type given below.

7C3457EF-0000-11AA-AA11-00306543ECAC


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