Apple's New Sinister Feature
medium.com
medium.com
It could hardly be called anticompetitive for Apple to require SSO in apps that already provide third party SSO. This has been discussed on HN recently. App developers who use their own client authentication and no third party SSO are unaffected.
I’m not defending Apple, I’m highlighting your fear-mongering to sell your service. If you have anything other than baseless accusations, please share.