That's only if you are making a browser and want to distribute it. There's no way that Google is going to mail howlers to end-users who use widevine with chromium forks.
Because you would need to get a widevine license from google. Mozilla was able to do that, but I know at least one chromium fork had it's application for a license rejected. It technically is possible to copy the widevine library from a chrome install to use with chromium, but non technical people won't know how to do that.
Widevine works with chromium just fine - by which I mean that like with everything chromium it gets broken without apparent reason every now and then, but after some time they fix it.
Apparently someone has, because I was using Chromium on Linux to watch Netflix several years ago without any trouble (this was before Firefox got it working).