The discovery that AMD Threadripper actually has four dies, just as the EPYC CPU, came when der8auer managed to delid the CPU and discover that there are actually four dies under the IHS.
While there were plenty of rumors floating around and a vague possibility that these can be enabled, James Prior, AMD's Senior Product Manager, went public on Twitter, saying that the two dies in question are indeed dies but use a different substrate and have no path to the operation, describing them as "basically rocks".
Threadripper is not a Epyc processor. Different substrate, different dies. 2 dies work, other 2 have no path to operation. Basically rocks.— James Prior (@cavemanjim) September 17, 2017
While this means that these are not dummy dies, at least not theoretically, AMD moved to describe them as dummy instead of inactive just to break away from all the drama and confirm that they can't be enabled in any way.
Hopefully, this will put an end to all the rumors as if there was a way, we are sure that someone would have managed to make it work by now.