I may have been born under a rock, but does Qt have java capabilities? Wonder why GTK is listed but not Qt. Unless I'm like two decades out of the loop, which is possible :)
I think they were just stating why they were considering it as an option - or maybe why they really weren't. I think the same argument applies for any UI toolkit - it makes sense for them to just use libwayland instead.
Building a toolkit from first principles is fun, I'll admit.
I toyed with using libxcb and ctypes to make a pure python GUI toolkit once. Got as far as rendering on a screen and and event loop. Then got bored when I realized how much work it was going to be. I suppose someone could do the same with libwayland :)
They were using GTK's Wayland implementation as a reference, I think that is why. Though a Swing application running on Wayland wouldn't really care about the underlying DE anyway.