Well they do go into some depth about how dragons are not really gods even though people worship them. So you can't expect them to fully obey the rules of gods. Dragons seem to have multiple existences all at once so I think this fits in with that idea, the avatar of Tiamat is just the echo of Tiamat in this material realm there can be others at the same time in other material realms. Which is just how dragons are.
I like that mythic actions are really breaking out of the Theros book, I think they make for great boss monsters.