Yes, they got one thing wrong though it's already been corrected in Canonical's design document.
But it seems to me that the main problem Canonical had with Wayland was that they wanted to run their stack on devices that only had Android device drivers. As far as I can tell Wayland relies on things like KMS to function, and at the very least Firefox OS has no plans to use Wayland because they don't think they can get it to use the Android drivers they're planning on using.
That seems like a very good and practical reason to me. As far as I could tell, Wayland is very much tied to KMS, and not interested in doing otherwise.
But it seems to me that the main problem Canonical had with Wayland was that they wanted to run their stack on devices that only had Android device drivers. As far as I can tell Wayland relies on things like KMS to function, and at the very least Firefox OS has no plans to use Wayland because they don't think they can get it to use the Android drivers they're planning on using.