Incorrect. I can watch 4K on my PC just fine using Edge or the Windows App, you also get 4K with Safari.
Server side Netflix has nothing to do, just send out the encrypted stream if asked by a client.
(Netflix do the same thing on MacOS with Apple signed drivers, backed up by hardware based security.)
Including Widevine. Widevine Level 3 is a software based solution that can be used on a variety of platforms, but the space for Widevine Level 1 requires OS and hardware integration (which doesn't include Windows for example, but is available for Android if the OEM does the requisite work).