"I can't understand how he can support him" doesn't literally mean "I do not perceive the facts", it is "I find it incredible that he holds weights on various factors such that he made a list of pros and cons, calculated the net value based on the factors' weights, and came to the conclusion that supporting Trump is net good in this situation".

For example if someone's weights are:

disruptor, 100

shows signs of insanity, 0.1

then I can "understand" that they support DT. I can't understand why they choose to weigh certain factors like that.