The distinction between instantiated and uninstantiated mathematical structures is not obviously meaningless. The Tegmark hypothesis is that this distinction is meaningless. Since this meaninglessness is not obvious, the Tegmark hypothesis is nontrivial.

I'm making a distinction between saying "physical world is a structure" and "physical world has structure": the first form seems to demand something unclear, and the latter seems to suffice for all purposes. Suppose things may either exist or not; but structure of things is abstract math, so it does seem clear that the properties of a structure don't care whether it's "instantiated" or not: the math works out according to what the structure is, regardless of which things have it. And since we only reason about things in terms of their structure, a distinction that isn't reflected in that structure can't enter into our reasoning about them.

(It might be possible to cash out "existence" of the kind physical world has as a certain property of structures, probably something very non-fundamental, like human morality, but this interpretation seems unlike the kind of confusion the argument is meant to counter.)