When you go to:
https://www.qt.io/product/features
And select only LGPL features, you can see that QtWayland is not offered. That one is only under GPL. There is more stuff that is not available under LGPL. That would simply mean that all software built on top of QtWayland needs to be GPL too.
Other option is to use the Commercial License, which is quite costly. Every year that Jolla has to not pay that license fee is a nice saving.
Plaese be aware, I am not a lawyer 
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