It isn't intended to stop a direct lightning strike dead in its tracks, that is a pipe dream. But it doesn't claim that, either.
This is a gas-discharge transient protection device which fires when the surge voltage exceeds some predetermined level. It protects against high levels of static electricity and some electromagnetic pulses which result from "nearby" lightning strikes, not direct hits.
Still, this is very useful protection since a direct hit is much more rare than an indirect one.
Alpha-Delta Communications has had their "Transi-Trap" devices on the market for decades (http://www.alphadeltacom.com/tt3g50.html
), long before the MFJ product, to do the same thing.
One problem with these little protectors is that in order to fire at a sufficiently low voltage to actually protect equipment, they cannot handle a lot of transmitting power (or the transmitting RF voltage will fire them, instead). So, most of these devices are rated for low power operation (maybe 200W or so) only. If you run 1500W output, especially under some mismatch conditions, you could be in trouble with almost any of the gas-discharge protectors.