Thanks for the reply. I am likely not the right person to ask Jolla as I am not sure how to raise the right question and how to deal with a possible answer...
I guess this should be a new thread in together.jolla.com.

Regarding Jolla C, indeed afaik only Jolla C is using Bluez 5 while other devices only have libraries that could be activated but still by default use Bluez 4. However, as you can imagine, Bluez 5 should be coming to all (most?) devices at some point.