I suspect there are a lot fewer customers for Amazon WorkDocs / WorkMail / WorkSpaces than for Microsoft's equivalents, though. I've certainly never heard of anyone using them.

In that scenario, the Microsoft numbers are even more mysterious as OneDrive for Business just another non-billable component of SharePoint that's consuming that storage, which AFAIK isn't Azure storage, but in dedicated Office 365 nodes. The Amazon Drive service AFAIK is just a frontend to S3.

The interesting thing about O365 is that it combines a very valuable licensing cash stream with a variety of services.

The story that Microsoft SEs used to sort of tell a few years back was that Microsoft needed so many spinning disks to support Exchange workload IOPS that they had ridiculous amounts of "free" storage available for low-IO workloads like SharePoint/OneDrive. No idea if that was true years ago or is true today.

Don’t think Amazon is as much as a competitor here. However, Google Cloud does roll in all GApps revenue into their Cloud reporting

Office is one of Microsoft's most lucrative lines, whilst I doubt that most AWS users pay much attention to WorkMail/WorkDocs. Many big companies play games with reporting numbers, and this is just a rather transparent example. Unfortunately, the fact that it got the Forbes story rewards that this kind of trick.

I think Dropbox uses their own infrastructure.