Dear John,
Greetings! Thank you for posting in the Microsoft forum.
As you mentioned “I found the support article, but I can't get the Export menu item to come up” I suppose you must be referring to this article, right? If so, this article applies to personal OneDrive.

For the notebooks stored in OneDrive for business and SharePoint online, there is no direct way to export them but there’s a workaround. For this, you will need a Windows OS with OneNote app (formerly known as OneNote 2016) installed.
If you get your hands on a Windows computer with OneNote app, you can follow the instruction from the following thread to move notebook from one OneDrive for business account to another.
Transfer OneNote notes to new Office 365 Subscription - Microsoft Community
Besides, I understand it is difficult and time consuming to move notebooks this way from a business account and in this regards, I humbly request you please consider sending your feedback to OneNote Feedback Portaland let the related team know about your requirement. This is the best platform to let related team hear from you and make the products and services better for you and others. I found posts in Feedback portal where other customers have requested the same feature. You can vote and provide your feedback in the following post.
Linux Support · Community (microsoft.com)
Export and Import OneNote notebooks · Community (microsoft.com)
Note*: In case your organization has blocked, you might not be able to access feedback portal with your work or school account, you can use your personal Microsoft account instead to access.*
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Neha | Microsoft Community Moderator