I assume they are deprecating "motherboard" as a gendered term, but the replacement of logic board also replaces a bunch of other names for the same thing.

The style guide contains other updates then just "non-inclusive language" [1]. I think the original comment mistakenly included logic board (Apple uses the term logic board instead of mother board/main board).

[1] https://help.apple.com/applestyleguide/#/apdaf2bc3367

If you're referring to the mandate to say "logic board," I believe that is just a stylistic choice, not a cancellation of "main" in the sense you're thinking of.