I highly doubt that someone on OneDrive has KPIs that performance on Windows is X better than performance on another OS. Maybe if both teams were in the same org, then I could see that as a (n albeit very slight) possibility.
Not knowing the code behind it, but knowing the history of the OneDrive for Business client, I could see a few ways this could happen. The OD4B client has gone through a bunch of iterations before being merged with the regular (public) OneDrive client, so along the way, there could have been optimizations added on that were only available to the new merged client. Due to constraints, this could have been done using a simple UA check. These optimizations could have been merged down to the OSX/Linux clients (not sure if they're using a merged one there, or still the separate ones) but since different teams work on the clients (most likely different teams on each client) and server, the Linux client team never told the server team that the it also supports these optimizations.
Easily could see the above scenario happening, it's not completely crazy when talking about big companies that teams don't have the best insight into eachother's work.
Disclaimer: Work on Microsoft. Not on Windows, not on OneDrive, completely different part of the company.