Those are just the claims the video makes, I didn't watch it again before posting it so I may have misrepresented it slightly but if you watch the video that should be what is said in it - though I do use DX12 when referring just to Async compute features which is somewhat disingenuous - The video claims that Async fixes the utilisation issues that DX11 has.

I don't necessarily agree with what it said, it's just the only source which I could find which claimed that there's an advantage/disadvantage from having more weaker cores than a small number of stronger cores. My understanding has always been that graphics rendering is embarrassingly parallel so the core count independent of other factors is meaningless, not saying that's true either, that's just what I've always thought.

But since this is all about the core claim I guess I better get back on it. The R9 390 has 2560 cores and 40 compute units. The GTX 970 has 1664 cores and 104 texture units. So it's simple to see that AMD has more, but slightly weaker cores that has lower utilization than that which Nvidia manages to achieve. DX12 removes some of this bottleneck put on AMD GPU's, Nvidia allready has very well optimised GPU's so not much overhead.

Which is what the video claimed, that AMD has issues with large core count GPUs and DX12/Async fixes some of them so you get better core utilisation on the big GPUs. It kinda makes sense as AMD does have verifiable driver overhead which is fixed in DX12 and - again - you see massive gains when running an AMD GPU in DX12 over DX11 when paired with a lower end CPU - like an Athlon or FX CPU - but you don't get the same gains from DX11 to DX12 because the Nvidia driver doesn't have the overhead that the AMD one does.