“Revoking their clearances (which is what the President is considering) is much more serious.”
Not serious at all; just just. When you leave a job you leave your papers and lock the door behind. Your clearance is part of your job and belongs to your employer in a just world.
In the current realm, a revocation IS much more serious than inactivation. Revocation means your clearance is removed completely, and should you require a clearance again, you would have to go through the whole process from square one. A revocation would also make it much less likely that the person would be able to pass a subsequent clearance investigation.