[Brightcolours]

It's really the distortion correction that makes the difference here. The barrel distorted corners are stretched out. With pincushion distortions you tend to keep the corners but stretch out the middle part (alternatively you shrink the edges and scale up the whole image. 

If you look at how much gets cut off at the edge, and then look at where the corners will end up, you still are above 3 stops + worth of vignetting. So, I still say the vignetting correction is the main factor here.

That digital correction thing doesn't seem lossless

Tried canon 750D plus 15-85 at 15mm versus Sony a6000 at 16mm corrected or uncorrected Canon is by far better, I can post 100% crops here if you want. But we are comparing a tiny kit lens versus a decent 800$ lens. So it's an unfair comparo results are to be expected.

I can try versus kit 18-55 if you want

Of course it is not lossless. That is not something you need to show, because everyone knows that?