One could argue in every case other than perhaps Bellard that while maybe some programmers are 10x more productive in spurts than other programmers, they average out to a similar productivity in the end anyway. Sure, Eich created JavaScript in 10 days or whatever the story is, but as far as I can see (and I mean no disrespect to Eich here), those 10 days were an anomaly in terms of his usual output.
I can only really speak for my own experience where I do often have bursts of creative output that are extremely productive which are then often followed by days or even weeks of relatively slowish slogging maintenance work.
Am I capable of "10x" work? Yes. Can I maintain that "10x" 100% of the time. No, certainly not.
I think Joel's Salieri/Mozart point is the far greater one than whether anyone is "10x" productive -- some developers are simply capped at a certain level beyond which they lack some combination of the basic creativity and/or fundamental software engineering knowledge that would allow them to perform some task outside of their routine. If all of your developers are of that type, you're pretty fucked unless you're just producing run of the mill CRUD apps all of the time.