The point is that any fluctuation at all means the Bitcoin doesn't represent a specific unit of computing power.

If the dollar is just a name for a certain weight of silver (as it was in 1780) then any fluctuation at all makes the label "dollar" meaningless.

This may seem strange, but let's use a different unit: the gram. If I promise to pay you 10 grams of gold 2 years in a row, and I give you 2 different masses of gold, then either I've cheated you or "gram" is just a measure of whim (or both, especially if it's my whim).

So if "Bitcoin" is a monetary unit that represents 10 petaflops of compute, and then next year it represents 8 petaflops, it's not really a unit at all. Similarly, a "dollar" that represents 10 grams one year and 8 grams the next isn't actually a unit.