Can an editor examine a PhD student whose manuscript is under review in their journal?
In the US system, my main concern in such a situation would not be conflict of interest in the thesis examination but rather would be conflict of interest in the journal reviewing process.

In the US system, a good advisor generally ensures that the examining committee are people likely to approve if the student does good work (e.g., no philosophical enemies) and ensures that the student does not go up to defend until they are certain to pass.

Members of a thesis committee are thus typically either members of the same department or close external collaborators, who should typically be barred by conflict of interest from handling a paper from that student at a journal in any case.