Whoops, you're right, I brain farted the wrong word.
To be clear, though, they were still wrong. `f(x)=2x` has the property they described of consistently giving the same input for a given input (if you pass in 1, it will always output 2), but it is not idempotent because f(f(x)) does not, in general, equal f(x).
To be clear, though, they were still wrong. `f(x)=2x` has the property they described of consistently giving the same input for a given input (if you pass in 1, it will always output 2), but it is not idempotent because f(f(x)) does not, in general, equal f(x).