This guys is not wrong.. but seriously what did republican strategists expect? Trump is a vulnerable narcissist... the last few months in particular the rhetoric at his rally's has gone off the deep end... he has the attention span of a mesqiuto, he said he wasnt going to prepare and he has at best a surface level understanding of policy ("build the wall!")... seriously what did they think was going to happen?
I think a lot of it stems from people assuming Trump is a normal functioning person whose behavior can be modified if only you tell him like this or like that. In reality he is a narcissistic sociopath who cannot be reasoned with. Just listen to the way he talks and the way he lies - that's not a learned behavior that can be adapted after taking a loss. He's been talking that way for decades, through multiple scandals and bankruptcies. The biggest, the strongest, the most successful, nobody has ever seen, nobody has ever done, people are saying, etc. But every 2-4 years republicans pretend like "if only" he can stick to policy he can appeal to Americans. Not happening.
He got one fluke win 2016 against a historically bad candidate. He's not a world beater, and whether he wins or loses in November I'd bet money that he'll end up getting around the same 46% he got the last two times.