I find the GADT syntax used in the article hard to follow... Normally this is what I see, straight out of Jannis Limperg's insanely helpful compilation of language extension examples[0]:

    data RestrictedMaybe a where
      JustInt    :: Int    -> RestrictedMaybe Int
      JustString :: String -> RestrictedMaybe String
      Nothing    ::           RestrictedMaybe a

Maybe the intention was more of a "from scratch" implementation (without using extensions) but the above syntax is a bit easier to follow in my opinion IMO.

It might also be a good idea to make note of where the polymorphism is happening (the "a") and note how it's ok that the a is "resolved" in some of these constructors but not in the others.

[0]: https://limperg.de/ghc-extensions/#gadts