I can't vouch for this extension in particular (because I haven't tested it), but I've used and written similar extensions myself and can confirm that the concept is legit.
The problem here is requiring hardware-attested DRM: Widevine L1 on Edge on Windows, and Apple FairPlay on Safari on MacOS. The only way to get hardware attested DRM is via browser specific (i.e.: native code) support that interfaces with the OS & GPU drivers. You can't get there through an extension.