Yet I'm starting to wonder if it's also the case that people with substantially low Agreeableness might also be under-served in different ways.

Low agreeableness makes it hard to even hear social advice properly. (It's hard enough for males to accept advice even when agreeable.)

And alternative social advice (e.g. from PUAs) often is designed for high-Agreeableness males, and emphasizes acting "high status," being "the prize," and "not giving a crap."

Surprisingly enough each of these three are still important for the low agreeableness/low status males to learn. It is just harder to explain which specific skills it would take to develop these attributes. Apologizing whenever someone else disapproves of you is not actually all that much different to attacking whenever someone else disapproves of you. It signals the same underlying insecurity.