At least that's what I do when I don't have the time for audiobooks. But maybe I shouldn't generalize from my own experience, as everyone is different. For me, audible + the occasional torrent works quite well, and I wouldn't like to use the Spotify interface for audiobooks anyway, considering how bad it is for that use case.
I checked out the FAQs on that page and apparently the 15 hours is just included with the Spotify premium subscription and that let's you listen to 15 hours of anything in their catalog.
Separate from that, you can also purchase books outright and it seems listening to those purchased books won't count towards that 15 Hours.
In that context, those 15 hours are actually kinda appealing to me since it would let you listen to more than the 5 min sample of a book you get on Audible before deciding to outright buy it or you could use that time to listen to a couple shorter books every month without having to purchase it.
Think I might have just convinced myself to look into it a bit more lol (I'm curious how good their catalogue is compared to Audibles)
I use Storytel myself. It's available in many countries. I think it's one of the best services out there for the unlimited listeners.
I use libro.fm. It doesn't have a subscriber model, but they give you DRM-free audio files.