recognize that the OkSt kicker had kicked the ball up into the air rather than down into the dirt.
There's a reason why all onsides kicks are either kicked down into the dirt (where they then bounce back up into the air) or squibbed along the ground: to nullify the opportunity to call for a fair catch.
But the OkSt kicker kicked into the air giving TT the opportunity to call for a fair catch, which they took. Once a fair catch is signaled, the kicking team has to allow for the receiving team an opportunity, which they did not. Hence, the penalty and the ball being awarded to TT.