Why is this value constructor not hidden?

Question

Refer to https://wiki.haskell.org/Smart_constructors and module code:

module Resistor (
         Resistor,       -- abstract, hiding constructors
         metalResistor,  -- only way to build a metal resistor
       ) where

 ...

When I try this in ghci, I am able to call both metalResistor and Metal. Metal was supposed to be hidden. What should I do to properly hide the Resistor value constructor?