Each die provides 512GB memory access.
Threadripper supports 2048GB memory, just like the EPYC.
For this to be true, combined with the last pic in above article, means that all 4 dies must work, to provide 2048GB memory for the Threadripper(and EPYC).
So, its pretty clear that all dies functions, but some might have damaged cores that means they cant use all 4 dies in all EPYC. But like EPYC, there are lower versions that doesn't provide 32 cores (4 dies), but they still provide 2048GB memory support.
So... for both to be true, a die can be none functional(as in all 8 cores works), and still provide access to the IO/mem, with the working ones.
All it takes, is that AMD make some of the cores working on the dies, and doesn't provide the core power of them, but takes the routing power of them.
In conclusion, which the video support, there isn't a thing such as a dummy die; at tops something AMD classifies as "not fully working die". Its just a way to make more profit out of EPYC CPU.
And yes, of course, all this leads to what the video also ends up at; A 32 core Threadripper will come, if demand is strong enough.
Better question will be; is it possible to re-open some of those dies to use their partly working dies, so you maybe get maybe up to 8 fully working core more pr die?