Divestment could be a punishment in some circumstances. E.g. if Congress passed a law requiring Elon Musk to divest himself of X as punishment for purportedly violating the Securities Act.

The difference here is that Tik Tok is not being accused of a crime and is not being punished for some crime. It's applying a restriction on foreign ownership not to punish Tik Tok for some past act, but because Congress is worried about the risks arising from that ownership in the future.