This clearly does not hold in Germany. There, you often can earn much more money with a vocational training than with a humanities degree, in particular in the current messed up job market.
On the other hand, people with different education levels often have very different psychometric traits (similarity here is very important for the relationship to work).
> Similar/compatible hobbies? Guess what, also a proxy for income!
There do exist some hobbies that are somewhat correlated to social classes (think of who plays soccer/tennis/golf (replace "soccer" by a different sport if you live in the USA)), even though these associations are much less pronounced than in former days.
On the other hand, for example, a person who loves sports climbing or parachuting will likely have very different psychometric traits from people who are into stamp collecting.
Thus, in many cases, hobbies are thus rather a proxy for some psychometric traits.