It was a poor over from Tom Curran. He (Tom) did an excellent job till then, conceding less than a run per ball and picking up important wickets. But Dhoni had something else in mind. And the DC captain just looked clueless. Personally I thought that it was better to go for Anrich Nortje or Kagiso Rabada in the last over, because Tom didn't had many outings for DC this season. Anyway, CSK is just one win away from the trophy. And that will be complete turn around from their pathetic performance in 2020.
Some might say that Dhoni played the 4d chess. He promoted Thakur up in the order just to make Tom Curran look good so that he can bowl final over.