If Do Kwon decides to visit the US and hasn't complied, he may find himself getting arrested. If his company decides to have assets in the US, they may be seized. However until either of those things happen, which they won't*, then the US has zero teeth in this.
* assuming he doesn't visit the US again
Wrong. They served the subpoena first; they need no permission to do that. Then SEC sued to enforce the subpoena, and the SEC prevailed in their suit.
Then Do Kwon and Terraform applied to have enforcement of the subpoena stayed by the trial court pending appeal, and failed; they then attempted to get the US Court of Appeals for the Second Circuit to stay the subpoena pending appeal, and failed, and currently are in the process of litigating the case before the Second Circuit.
To portray this as a case where the SEC has failed (spefctacularly or otherwise) to establish jurisdiction is false.