It is almost certainly a mistake, as the expression "quick as silk" not only makes no sense (silk has no speed) but it cannot be found anywhere else.
This is not strictly a "malapropism", as it is unlikely one would hear "flash" and think it was "silk". A malapropism is when someone gets an idiom wrong by substituting a similar sounding word, eg "he danced the flamingo" (instead of "flamenco").
Neither it is a "mixed metaphor", as this is a statement that contains two incompatible comparisons, for example "we need to iron out the bugs" (who irons bugs? It should be iron out the creases or crush the bugs).
I think what you have on your hands is a Malaphor - an incorrect mixture of two idioms, or clichés, also called an idiom blend.