By the same people who wrote NUnit.
They rewrote it to propose a new way to test, rather than break NUnit.
Should you switch? Probably not: millions of lines of code have been written using NUnit, so NUnit not going away. If you are wed to Setup/Teardown, then it's probably not worth the effort to switch. But if you are not using Setup/Teardown.
[Fact] is good to go as a test.