Ironically, we don't know what the penalty for non-compliance is because we don't know the non-profit's gross receipts are for its most recent complete tax year. If they are over $1 million, which is plausible, the daily penalty is $100/day not to exceed $50,000. See 2022 Form 990 Instructions, page 81. Not that this really matters.
The crux of our disagreement is whether they have the right to refuse entry. They can [unlawfully] deny a request I suppose and just choose penalties over compliance, but when it comes to denying the ability to lodge a request in person with the associated "immediate" timeline, there are First Amendment interests at stake and I think the federal constitution, if not federal preemption, wins.
In this specific instance, it wasn't clear to me what, if anything, the security guard had done or intended to do with my name and ID information (which is not needed to file a request for tax forms). So I didn't feel as though a specific request had even been filed. He wouldn't provide his name, he wouldn't allow me to speak to anyone with any knowledge of the issue, and I had nothing in writing. From my perspective, OpenAI prevented a request from being filed after its accountants said that they had the documents but that the non-profit's offices were the proper place to lodge a request.
On top of all of that, the 2022 IRS Form 990 instructions (page 79) refer to "the room" and the requirement that the member of the public "must" be allowed to "take notes freely." Outside on the sidewalk is not a room where you can take notes.