Effectively yes, that line says that there is a 10% chance that it will do the calculations for a lightning strike if it is raining, but its a little more completed then that:
This basically says that the top of the lightning Rod has to be higher then 128 blocks high in order for there to be a chance that lightning will strike when it is just raining, beyond that the probability remains the same.
Good to know, I was under the impression that it ONLY happened in lightning storms.
Now Forestry lets us manipulate the rain...
Oh god! Here come the perpetuum machines!