Bevan Fransman slotted the winning kick in the shoot-out after Musa Bilankulu cancelled out new Sundowns signing Fiston Abdul Razak's 105th minute opener at Lucas Moripe Stadium.
Mosimane said: "I think after we scored in the last six minutes we never settled and obviously the opposition went all the way to get an equaliser.
"I was not happy with the foul that resulted to the goal. But it happens in the game.
"I think we were never under threat so much. There were one or two chances in the first half.The one hit the side net, it looked dangerous.
"But above all, I think we dominated, we tried to win and force. But you know when you go to penalties, you will not be lucky all the time.
"In the Nedbank we won and tonight was not our night. We should accept and congratulate Bloemfontein Celtic."