Some reasons:
1- He looked great, but it was just one inning.
2- If Kikuchi and White and other depth options suck, we will probably see him Tiedemann in 2023. But if Kikuchi starts performing like a respectable #4, then the rotation would be full barring an injury or a trade.
3- While Tiedemann could be better than most of the pitchers in the current rotation, he is not on the 40 man roster. There is some asset management aspect to the decision.