What is meant is that services explicitly short change Linux users by restricting resolution based on client in a fashion that isn't trivially circumvented by fudging the user agent.
For instance a given content may be available in glorious 4K only via an app on a TV, 1080p via a windows or mac machine, and dogshit 720p that actually manages to look worse than old SD on Linux.
This is basically true of every service with Youtube TV being the least bad and showing at least reasonable quality. In theory showing lesser quality streams to less locked down platforms is supposed to make piracy harder. In reality popular shows are always available same day anyway.
Ultimately if you want creators to get paid and to enjoy the highest quality on Linux you need to both pay for and pirate the content and watch for instance on jellyfin.
