It's not really well explained in the article, but the argument against Symbols is that you (client code) have to store them somewhere for reuse in different calls.

That is that, you could do...

    useState("someID")

...and somewhere else (* or in the same place but on a different call) again...

    useState("someID")

...and this indeed refers to the same item. But using a Symbol, you need to first create it, store it somewhere and then use it. That is, you can't do this...

    useState(Symbol("someID"))

...because this will fail through different repeated calls. Instead you'd need to first...

    let someSymbol = Symbol("someID");

...and then...

    useState(someSymbol)

Or, alternatively, use Symbol.for("someID"), which then has both problems: creating the symbol first and clashing of identifiers.

While the article does not explain this clearly, the example used alludes to this in an indirect way.

Personally I do think that this would be a more desirable sacrifice to make than restricting call order, but the React team thinks otherwise, it seems.