That is correct, the AC must be WAAS enabled. WAAS never gained any traction in the US, and airlines never saw the benefit to spend the money to enable it.
You also have to remember that WAAS is an augmentation, rebroadcast of the correction factor, that a WAAS enabled receiver can use.
GBAS is far more useful as an augmentation, and can also broadcast the Approach for the AC to use.
While the cell phone may be able to receive WAAS, as most GPS antennae can, very few, if any can use it.