Definition of "hobby"- "An activity done regularly in one's leisure time for pleasure."
Even if one hosts irregularly and doesn't depend on it for a major source of income, it most certainly doesn't fit the definition of a hobby.
There is nothing pleasurable about cleaning other peoples' hair out of the shower drain or trying to get the bloodstains they left out of the linens.
Saying that just because a host doesn't do it as a full-time money-maker, aiming for full occupancy, relegates it to a hobby, is trivializing the work that goes into it and demeaning those hosts.
It takes the same amount of time to turn over a room if you do it daily as doing so less frequently. Dealing with bookings and guests is no different.
And a hobby is something people would do regardless of whether it involves financial gain. Sometimes a hobby can become a lucrative business, for instance a knitter might be wearing a hat she created, someone admires it and asks where she bought it, and suddenly she's getting orders for hats.
But the basic nature of a hobby is that one would do it basically for themselves no matter what because they find it enjoyable and relaxing.