“How is the truth a low blow?”
Do you really need that explained to you????
How about an example? If I said “Trump obviously maintains a ridiculous combover and chases young bottle blonde bimbos because he has immaturity issues”, that would both be true, and also a low blow.
It implies it was is fault. In fact we all know the security lapses during the 90’s is what led to 9/11. Not on someone who became President just 8 months preceding. Don’t be such a chump.