First off, I think Avatar is DCA bound. They wouldn't want it to eat into Galaxy Edge's attendance.
I think the prime reason Disney is being coy about Avatar coming to DLR boils down to one word: Politics. With Disneyland Forward either a done deal or close to the finish line, maybe Disney is worried that if they were to come out and confirm that its a land or where it is going, the City of Anaheim would find something to take issue with (like the sightlines) and that could very well derail all of Disneyland Forward. Get DL Forward approved then you can announce it at D23 2024 and go from there designing a land that fits in with whatever guidelines are laid out in DL Forward.