You're right, it doesn't have to apply to everyone, but some acknowledgement by the OP that his story is but one sliver of experience would be a good idea. After all, he's selling a book and an app/extension.

What’s the utility of life advice that’s meant for a general audience, but can only be utilized by a specific niche?

We’re all trying, mate. Sometimes one of us writes down their experience to see if it can help anyone else. I am not sure if there is any advice that is truly “for everyone”.

It either works for you or it doesn’t. If it does - great. If it doesn’t - I hope you find something that does.

The only way we’ll know is if we share our experiences. I don’t see any value in critique here.

This should be pinned to the top of every forum thread in the internet. Well said.

Sure. It’s also a good way to filter out advice that’s less applicable to a larger audience. Perhaps it’s aspirational advice of sorts for those who can attain the material conditions of the one bestowing it.