I think this is just caused by Jolla having insufficient resources for updating individual packages - because even if you update just a single package there is still a lot of stuff the individual update can influence that you need to test before releasing it to end users. And that can add up on QA resources quite quickly.

On the other hand just QAing it all at once before a batch update can save a lot of QA time as you just need to run all the tests once.

This is still hopefully just a temporary thing, as you should really release at least security updates as fast as possible, not make them dependent on non-critical bug fixes and feature additions. And major Linux distros manage to do this just fine.

But compared to other mobile platforms I guess even batched but regular security updates are a huge improvement.