Of course this is rumor and speculation, but for me the thing that gives it credence is that it follows common sense.
If the family share plan worked as we all dreamed, it would have been a huge bullet point feature that MS would be shouting from the mountains, especially after Sony had their conference. That was not the case. Also, if publishers don't like used games, how do you think they like the idea that 1 copy sold could be freely distributed amongst 10 people with minimal restrictions? I just don't see it.
And I don't think this discussion is irrelevant just because it is hypothetical. The fact that MS left this open enough for there to be a discussion in the first place speaks volumes.