Three major releases behind, but I certainly don't blame them for locking down. Epic have trended toward favoring features over fixes in the last few releases of UE4.
Stuff like releasing a new event sequencing framework with showstopper bugs that prevent it from being production-ready, or breaking roomscale for Oculus Rift and then backlogging the bug ticket for an indefinite period of time.
Since PS4 uses the older version of UE4 parity of Switch will be even better now,
Switch version won't be much downgraded it will be more similar to PS4 version since it got the newer and updated optimised UE4 right?
But strange it's not using the latest UE4 which is 4.19 which has many optimisation improvements for PS4/XB1 maybe Switch too but no mention of it here:
http://m.neogaf.com/showthread.php?t=1443233
https://www.unrealengine.com/en-US/blog/unreal-engine-improvements-for-fortnite-battle-royale
4.19 isn't out yet, you can only get it via the still-WIP github branch. 4.18 isn't either for that matter- that's currently being released as preview builds. 4.17 is considered the current 'stable' release.
As for the bolded, that's a baseless assumption. Engine optimizations will help, but not enough to magically give the Switch parity with the PS4.
It's very difficult to upgrade the version of the game engine once you're at a certain point in development, since it can introduce a whole host of new bugs you need to find. It being on 4.15 tells us that development likely kicked into gear in earnest around when 4.15 released, or at least before 4.16 released.
To shed a bit more light on this,
in theory upgrading between UE versions should be relatively painless. You can test for new bugs and restore from source control if something's broken.
Downgrading when you discover a critical engine-side issue some weeks later on the other hand, is more or less impossible without losing work. Makes it too big of a risk.