That is an AT&T marketing restriction, not an engineering restriction.
I don't think that AT&T has ever allowed a U-verse installation in parallel with any standard DSL circuit (either from AT&T or a CLEC) at the same residential address. In fact, they will now not even allow two standard DSL circuits at the same residential address: »Edited title: AT&T only taketh away
I have also heard of instances where the new U-verse installation failed, and the customer was left with no internet connectivity because AT&T also refused to allow the old standard DSL circuits to be reconnected (but those were on-line forum reports that I have no way to verify).--
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.