> Could anybody explain how this isn't easily circumvented by using a competitor's model?

If the problem is "kids are using AI to cheat on their schoolwork and it's bad PR / politicians want us to do something" then competitors' models aren't your problem.

On the other hand, if the problem is "social media is flooded with undetectable, super-realistic bots pushing zany, divisive political opinions, we need to save the free world from our own creation" then yes, your competitors' models very much are part of the problem too.