I agree entirely that it would not be very reassuring, for the reasons you explained. But I would still consider it a mildly interesting signal to see if OpenAI would be willing to provide such an agreement in writing, and maybe make a public statement on the precise way they used the data so far.
Also: if they make a legally binding commitment, and then later evidence shows up that they violated the terms of this agreement (e.g. via whistleblowers), I do think that this is a bigger legal risk for them than breeching some fuzzy verbal agreement.
Really enjoyed the read—adding links with references to current day developments makes it tie very nicely with the recent past and present!
Minor typo: OpenMind should be OpenEye