Really warm? Nah... not possible... I quickly googled some specs on tasers and most of them operate around the 10-20 watt range. Thats less energy/heat than a cfl lightbulb, and the quick pulses coupled with only a couple seconds of use wouldn't heat the clothes up much more than turning a small cfl (60 watt equivalent, 13 watt consumption) on for about 3 seconds.
I do doubt the effectiveness of it shorting the taser out though. At that high of voltage you're likely still going to feel it even if the clothes did make a good conductor. But I doubt the clothes could be made into a good conductor with only a 10% blend. There's just too many other "insulator" fibers to keep the the conductive fibers separated.
I am pretty sure he meant that the clothing will make you warm the rest of the time? Maybe? That is what I assumed. Then again, what would I know?