I think it really depends on what the hobby is.

If you’re into an individual sport and you have private lessons, which are more common than you’d think, you’ll easily blow thousands of dollars per year just for half an hour or an hour per week. Almost anyone who is beyond a fairly recreational level in tennis, martial arts or fencing will be spending thousands per year.

On the other hand, 3.9k per year is a massive amount of money to spend on cheaper hobbies like hiking or painting.

It also depends a lot where you live, HN is very US centric and is full of people who are earning big money at tech companies there.