The idea is that you would apply a high voltage to just the cable, with nothing else attached. If it's just a cable, no harm no foul. On the other hand, if you start seeing smoke from the embedded electronics, you may want to use a different cable.
Unfortunately, that approach will become less useful in the future, as all USB-C cables that support USB 3 and/or high power contain a chip to advertise that fact[1]. (And then there are Thunderbolt cables, which look the same but require even more sophisticated electronics.)
You could also just use one of those cheap USB power/voltage monitors as the author does. Obviously a cable alone should not be drawing power.
Maybe an innovative company could add warnings for USB ports that draw power but have no recognized device. For the advanced ports with charging support and so on they should have power monitoring already.