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).
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.
Hah, I see where you're coming from. If your SOC has CPU, GPU, PCIe routing, and networking all packaged up, then the "main" board isn't doing anything all that main anymore. Unexpected sign of the times.