Based on the other footnote, I think the reference to "commercial users" is drawing a distinction between business Office 365 users (i.e. their own instance with a custom domain) and personal Office 365 users:
> Whiteboard currently supports collaboration within Office 365 tenants for commercial customers, and across personal accounts for customers with a Microsoft account. Collaboration across multiple Office 365 tenants is planned for future release.
If I'm right, anyone will be able to download the iOS Whiteboard app but only login if they have a business account.
No, anyone can login. What they're saying is, O365 Commercial Users [companies using O365, not personal accounts] can only collaborate with other O365 Commercial users, while Personal users can collaborate with other personal users.
Cross-collaboration [so, xyz@company.com collaborating with abc@outlook.com] will come later.
I was responding to the question asking if the iOS app will have to be paid for. Personal users may be able to collaborate but based on the other footnote, not through the iOS app.
> Microsoft Whiteboard is available as an app for all users on Windows 10 devices. For commercial users, the Whiteboard app will be coming soon to iOS