It's just a language argument, so there's no "right", but to me "side-hustle" implies money-making or at least career-building, as differentiated from, say, a "hobby" or a "pastime" which mostly costs money (or for some people, maybe includes enough transactional stuff to break even).

And then "side-hustles" became fashionable, so people talk about getting into coffee, or beer, or fixies, or ukulele orchestras, or scuba diving - as their "side-hustle".

I "play with" coffee making, electronics, drones, motorcycles, electric bicycles - none of them are a "side-hustle".