Yes, like you already speculated, 5.1 is not going to get any new minor release at this point.
The new Jolla Phone will ship with 5.2, and other devices will get that a bit later. We simply haven’t had bandwidth to test the other devices much, as the focus has been on making sure things work on the new device. And we will probably have our hands full with the feedback for the new device when it ships, so the release for the older devices needs to be staggered slightly.
So don’t worry, the older devices have not been forgotten, we just need to manage the amount of moving parts on the table at once. And a bit of good news is that we probably don’t need to drop support for the Xperia XA2 and 10 yet, unlike earlier estimated. So those would still get 5.2, with the updated browser engine and other new things. 