Ah, no, you skipped a step there. Before “Publisher presents Exhibit A, OGL 1.0a as their evidence they had a license” there’s the “Discovery” bit.
In discovery WotC’s lawyers will ask you to hand over a bunch of documents. These documents may hurt your case (even if you think they don’t). You will probably be legally obliged to hand over those documents (and will essentially auto-lose if you don’t). They may also ask for documents that you don’t have to hand over. Figuring out which is which will require an expensive lawyer. And if you can’t pass on the documents (they’re lost, destroyed, never existed in the first place), you probably need a lawyer who knows how to deal with that.
And of course discovery allows you to ask them for stuff - but what stuff should/can you ask for? Again: you need a lawyer.
OK so I'm not a US lawyer. I'm definitely not a litigator. I'm just an academic who teaches Law.
But to my mind, the legal issue in question would be: "Can WoTC legally revoke OGL 1.0?"
I have trouble seeing what documents the respondent could be holding that could possibly make any difference to that question.
I'm guessing the tactic for WoTC here is to throw up a bunch of squid ink on legally irrelevant evidentiary questions that do require Discovery? But surely a good lawyer for the respondent can draft a position where it's clear what the question at issue is? How can they force Discovery over an issue that's not in question?
Edit: Conversely in a case like GW vs Chapter House, one relevant issue is whether actual copying occurred. That's a question of fact, for which Discovery is relevant.
Edit 2: I guess they would want the focus to be on your copying of copyright material in the SRD, and how much was taken. That's a question for which Discovery is relevant. But if you don't dispute that you copied, then it goes back to the legal question of the licence.