Yes, the datasheet at least tells you the acceptable voltage range of En, and that it will disable the output. However, I don't follow how your quoted text defines the relationship between En and Vreg -- being the main point of my gripe. (To be clear, Vo != Vreg. They are two distinct regulators.)
On one hand, you might assume Vreg will be disabled -- it's a regulator, and you might want to have control over whether it's enabled.
On the other hand, you might assume Vreg will NOT be disabled -- it exists for control of the switching regulator, so it should be available to... control the regulator.
It took a failed prototype to determine that Vreg is in fact disabled when Enable it de-asserted. No biggie, but that would have been nice to know before wasting ~$20 on a board and some parts, and a couple weeks for turnaround. In hindsight, I should've just broken out the pins so I didn't have to re-spin the board if I guessed wrong. C'est la vie.