That has always been the theory. Not sure it has been confirmd but the NFL has deliberately gone after a single partner for Sunday Ticket almost from the beginning. They had to make some deals with the networks to get the games (for example, I think Sirius XM stll does not get games from the Titans).
The NFL is not dumb when it comes to marketing itself and its programming. They have made some missteps but not many.
Not only do they want to keep the value of the CBS/Fox contracts, but they control the quality of their presentation. They know that DirecTV gives them quality. They do not trust In Demand (and neither would I. Not only has In Demand been horrible at sending HD out; they are up to 7 channels but very few have more than 4 for baseball, et al. Plus I have seen the In Demand HD for EI; it looks bad even on Fios!). They have no inkling that Comcast, a hodgepodge of varying systems that, in many places, still has major bandwidth issues can deliver consistently. Plus they want a national footprint. Not regional. A single, monolithic prescense. That is the NFL. And only DirecTV can give them that (well, Dish could, but they are not a sports oriented provider).
They are very happy with DirecTV and would not want to dilute or confuse the product.