>But despite this, are you seriously claiming that sum types "do nothing" to achieve this null safety?

No. I said nothing about null safety. What I said is that “sum types by themselves do nothing to force handling of errors”. In fact I imagine that’s one of the reasons that Haskell uses exceptions for error handling in the IO monad. If Haskell had a non-raising function like

    openFile :: FilePath -> IOMode -> IO (Either IOError Handle)

then you could of course attempt to open a file without checking for an error:

    main :: IO ()
    main = do
        openFile “/foo/bar” ReadMode
        putStrLn “Did the file open successfully? No idea”

Sum types (by themselves) cannot be used to prove that all errors have been handled. In fact, formal proofs of this property using other type system features (such as linear types) are of fairly limited practical value, given that merely 'using' an error value in some type-theoretic sense doesn't necessarily entail actually taking appropriate action to handle it.