But if you solder on the lead, the lead will be longer.

Can be ok if trying to do table placing signs, but definitely a problem if you intend to throw the light and get a magnet to stick it to a lamp post or similar. Preferably, you don't want too much of the LED part to stick out outside the battery package or you may bend/short/break your device if it lands with the LED first or maybe sideways. It is quite likely that the units will spin in the air, so you can't expect that the LED will act like a sail or tail fin.

So the net result of adding a resistor may very probably improve the MTBF from overcurrent while introducing drastical failure rates from shorts etc.

I think they should continue to skip the resistor (the battery is claimed to light the LED for a week or more, so we can't talk about any huge currents for their real-life use) but tell people to keep an eye on the voltage and/or color and definitely not select a 1.6V red LED.