Brian McKenna is a public figure who people could easily have encountered in other ways.

In my experience many functional programmers, myself included, do generalize where they don't need to (e.g. writing a function to accept any F[_]: Foldable when it's only ever called with List). At which point asking them to justify the abstraction is entirely justified. Conversely in cases where fancy functional techniques really are improving maintainability it's usually pretty easy to demonstrate this by comparing two implementations side by side.