To chase your tail means to take action that is ineffectual and does not lead to progress, but in this sentence the person is taking productive action and making significant progress.
Consider the staetement ""But it's been so much fun watching you try productively and making significant progress," Briggs mocked, still keeping himself hidden behind his black magic.". Based on this statement, is ""But it's been so much fun watching you chase your tail," Briggs mocked, still keeping himself hidden behind his black magic." a true statement? Yes/no/unknown