This is true, but a simplification. Specifically, it doesn't distinguish cases where good advice is mostly impersonal (e.g. how to invest money) from cases where the best advice will be highly personalized (e.g. diet).
In many complicated fields, like diet, good advice needs to be individual. Learning enough about the field to choose the right advice yourself may take years. And it's prohibitively expensive to find what works best for you by trying everything. At best, you'll stick with the first thing that works moderately well.
So I propose category (4): the advice is valuable, not because it relies on nonpublic information, but because matching the right advice to each person is complicated (though based on public info, such as medicine). People who study the field, master it, and then give personalized advice add real value. Most importantly, to trust the advice of such people, you don't need to assume an extraordinary degree of altruism on their part. Ordinary situations like paying an expert for counseling may be sufficiently trustworthy.
Learning enough about the field to choose the right advice yourself may take years.
In that case, how could the expert possibly know enough about the field to choose the right advise for someone they only know through at-best several hour long appointments?
Lukeprog has done some great posts talking about how scholarship has benefited him personally and how to do it. However, he hasn't talked much about how to tell the good from the bad, and I think this is an especially big problem with it comes to "how to"s, rather than standard academic subjects.
The thing is this: writing on academic subjects tends to be dominated by academics seeking to gain status from their work. Not all fields have a strong correlation between status and making actual discoveries, but many (perhaps most) do. So for academic subjects, the big challenge when it comes to making sure you're not reading nonsense is to try to evaluate the soundness of the field in question.
When it comes to seeking out "how to" advice, though, writing on the subjects seems to be dominated by people looking to make money selling advice. That means that if bad advice will sell better than good advice, they'll give the bad advice. Even if their writing includes legitimate advice, they have incentives to give a distorted picture of the subject they're writing on if it will sell better (for example, making whatever they're talking about sound easier than it really is). In some cases, advice books end up trying to nudge you towards buying some more expensive thing the author is selling.
So: when you're researching a "how to" topic, how do you tell the good advice from the bad advice?