It didn't take 20 years to be discovered; PoC's for variations on this theme were found over 10 years ago. The details really matter for making an exploit: so

It didn't take 20 years to be discovered; PoC's for variations on this theme were found over 10 years ago. The details really matter for making an exploit: so the amount of speculation and what gets speculated, how caches work, how good timing is (and how large the difference between cache and memory is) etc. Merely knowing that the combination of speculation, caching, and timing have the potential to break through memory protection barriers is a far from enough to actually exploit that weakness. For comparison: it was widely known that sha1 had weaknesses, yet it took many years for somebody to construct two pdfs that actually demonstrate a hash collision.