This is hilarious. The voltage is what it is regardless of the tester being used. If someone is a "true" 8.4v on the nose and the tester being used reads high does anyone really think this person is really a cheat or trying to beat the system? Knowing how to quote a rule book and actually thinking about the consequences are not mutually exclusive concepts.
The rule makes sense in principle and is probably based on great reasoning. Just make sure it is measured correctly so people breaking the rule are punished, not the ones who aren't. Why give people even more reasons to stay home?