I don't think GP suggested that some people can't grasp the content of a mean message. GP specifically wrote, "the premise that you don't listen to messages that are too mean," and not, "the premise that you can't listen..." Usually, people would be would be capable of grasping a 'mean' message's content, but they reject it because of their own passionate reaction to the 'meanness' of the delivery.

The first thing GP suggests is that communicating a message more 'nicely' means more people will listen. So I'm not sure what your point is here independent of what GP has already written.

I suspect GP is emphasizing the importance of stomaching meanness because that's the harder (and more rewarding) thing to accomplish. It takes some effort to phrase and intone things more gently. But it takes a lot more effort to unlearn your own rapid emotional responses to someone else's tone... And successfully doing so is like having a superpower in social interactions. It starts to seem like everyone is driving drunk with respect to their own emotions: reacting instinctively when emotions are so strong that they drown out or dulled down all the other information.