There is no catch (in the sense of it being fake). It's cheaper because you're booking it from someone who "owns" it. It's like renting someone's timeshare at another resort, it ends up being cheaper than buying direct.
The only "catch" is that you can't cancel it or change the booking once you book it through David's or another company. You also don't get DVC member discounts (but you do get free hotel parking). Also, you typically need to plan your vacation much further out than originally because DVC rooms get booked up fast.
For example, we booked our two upcoming trips 9 and 10 months in advance.
We've used David's (Polynesian @ $17 per point) and everything went smoothly. You can still control fastpasses, add ADR, do magical express, etc. (You link your reservation to your
MDE app.)
We now just book straight from members for a bit cheaper. We create and sign contracts with them, and if you build a good enough relationship with the owner, you can sometimes edit your booking or cancel it last minute. But for a first-timer, I'd suggest using David's or DVC rental because everything is taken care of for you and they do the heavy lifting (hence why it's a bit more expensive).
If you have any questions, feel free to message me. I've learned a lot about this over the past couple of years.