To be Transhuman is to be something other than human. Just as we might expect a machine intelligence to seem alien to us and to consider us alien, we might also consider such humans who transcend their humanity to be as different.
The usual answer to this from would-be Transhumanists is that, if Transhumans are going to have all the talents and abilities, they are going to have the advantage in any war. Join up or find yourself on the losing side.
But Harris shows us a view of Transhumanism so revolting that one might expect the entire human race to rise up en masse and divert every effort to stamping it out if it should ever become more than an occasional curiosity. While most would-be Transhumanists probably do not plan on becoming cannibals, the whole point is that you really can’t plan at all on what a transhuman being would think is an appropriate way to treat traditional humans. Even if that potential transhuman being is the one that was once you. Just ask Clarice Starling.