Surely the argument being made here is for the best prototyping language, not the most familiar. If familiarity was the only yardstick nothing beyond C would even exist.
The argument being made is if it's necessarily efficient to prototype in a different language than the one that will be actually used, especially if one is more familiar with the target language.
In a world of options where an experienced programmer has had a chance to experiment with different languages the most familiar language is the best prototyping language. In a world where C is the only option programmers think about what other options they could imagine.