Some people are better at all types of programming than are lesser mortals

Are you making that claim, or suggesting that this is what the 10x thesis means?

Both.

(Dijkstra once claimed that "the use of COBOL cripples the mind". If true, it would follow that someone who is a great COBOL programmer would be a poor programmer in other languages.)

Amusingly, that does not follow. A great COBOL programmer completes his COBOL tasks in 1/10 the time of lesser folk, and hence becomes 1/10 as crippled.

you seem to be assuming ...

Where is that implied in what I wrote above?

It appears that I somehow misinterpreted your point and thereby somehow offended you. That was not my intention.

You begin by mentioning the problem of testing the 10x hypothesis, and then switched to the problem of trying to separate out "how variable the time required to complete a task is intrinsically". That is an odd problem to focus on, and my intuition tells me that it is best approached by identifying that variance as a residual rather than by inventing an ideal thought experiments that measure it directly. But if someone else has better ideas, that is great.