Here's a write-up from PetaPixel,
...and some info from Sigma about its modularity,
...and at last... I found some specs from Sigma (which I missed among all the noise before)

I must say that with 4K (2160p) and 4X slo-mo at 1080p, the video specs look interesting, too
And of course you need USB3. OK, the rolling shutter allowing only 1/30sec flash sync is a bit of a
challenge, but not insurmountable.

- - - Updated - - -

Woops! In the specs link from Sigma it says:
"Type Interchangeable-lens SLR Type Digital Camera " to them.