I stand by it being a non-problem in practice because the workaround is trivial, and the supposed solution adds complexity everywhere.
There isn’t even a universally agreed methodology for how to handle nulls in the relational model in theory.
It’s a semantic problem that can only be solved with reference to the requirements of a particular use case, and when you do that using explicit extra columns, the problem evaporates.
(Existence of a workaround doesn't turn a problem into a non-problem)