Jolla could well profit from announcing having found a spare battery supplier. It would have cost nothing but an official blog post and some testing as you correctly say. The fact they did not already do (since two weeks) tells me that some legal issues keep them from doing an obvious step from user perspective.

Could it be that Jolla is obliged to provide warranty for the whole bought package (Phone, Battery, Charger) and its combined use?
Publicly announcing compatiblity with another battery may take them into trouble if those fail, regardless the perfect build quality of e.g. polarcell. Maybe if their original battery fails in a Jolla Phone ( and hurts someone), they have a special insurence from the ODM?

Just wild guesses, but some legal stuff is the obstacle for sure.