To be more clear, they are recommending you not use this particular emitter with unregulated drivers and batteries where other emitters might work fine. That's because the voltage drop (Vf) of this emitter at working currents is quite low. That would cause the unregulated driver to give it more current than you expect. Many modern batteries could easily supply enough current to damage the emitter.
Unregulated (aka 'direct drive', or DD) lights, FET drivers included, have been around a long time. The cheap give-away lights are all DD. They don't even have a FET, just battery, LED, and switch. They depend on the internal resistance of the batteries and LEDs to limit the current. Many of them use 3x AAA alkaline cells, which have quite high internal resistance. Smaller ones use coin cells, which have even higher resistance.
By contrast, an FET driver is normally used with LiIon cells and high-power LEDs, which both have substantially lower resistance. In the past, the LED's Vf was close enough to the battery voltage that the fairly low resistances were enough to limit current to safe levels (albeit much higher than in a give-away).
But with today's super-low resistance FETs, high-drain cells that can provide 35A or more, and with every generation of LED having lower Vf than the last, it's quite easy to put together a DD light that will self-destruct in seconds.
That's what they are warning you about.
The way to "tone down" any DD light is to ensure that there is enough resistance in the circuit to limit current to safe levels. In the past it's been easy enough to find this resistance in the batteries, FETs, LEDs, wires, springs, etc. In fact if you go back far enough in history or cheap out enough, it's hard not to find this resistance in the components. A lot of the history of high-end DD lights is about getting the resistance low enough to get incredible outputs from the lights.
Or you could just go with a regulated light.
Others, like the Nichia 219b, cannot, and hence the warning by MTN. Wiring multiple LEDs together, like in a triple or quad setup distributes the power, so collectively they can handle a DD or FET setup.
