Nothing is Completely Indescribable For Certain
Or to put it differently: we can't be certain that something is completely indescribable. There may in fact be things that are, but to claim that something is indescribable implies that one knows one attribute of that thing, namely the attribute of being indescribable. But you should not be able to know any attributes of that thing if it is truly indescribable. So to proclaim the existence of an "indescribable God" doesn't make sense, and thus doesn't work as an argument for an inability to define God.