This is actually what hyperthreading is all about: cache misses. I missed that in the article. There are more things missing actually, but I guess it would be too much to explain it all in a single article. Things like caches, coherence protocols, prefetching, memory disambiguation. Registers are also much more complex because you have things like register renaming, result forwarding etc. In the end there are simply much less registers than memory locations, that's why you can build faster registers than memory.