Ihave seen plugs that work when tested in the traditional way (lay them on a grounded surface and crank away) work, but fail in actual use. Is that because of the increased RPM air combustion chamber pressure? No idea…..
Having said that, for the price of that unit, I’d be tempted to order. Assuming it works, could we watch a plug firing at high speed and observe the consistency? Compare it to a new one? Might be useful.