Which (real and trusted) browser would send "ms-office" in its User-Agent string? I'm not aware of any browser that would send a string like this - not even IE or Edge
Not even UA-Sniffers recognize "ms-office"...
I've found the same thread on TechNet, which you have already referenced (https://social.technet.microsoft.com/Forums/Azure/en-US/348c37d1-5e17-4578-bb89-d12a5390816b/list-of-useragent-used-by-microsoft-office-products?forum=Office2016ITPro), but I still doubt that there is a relation between protected or unprotected documents. The last time I've seen this was on a wordpress blog page.