If true, John McCain making a public announcement that Pres Trump is not invited to his funeral is a good thing for the Pres. It gets him off the hook for never intending to go in the first place and he can sincerely express sympathy for Senator McCain and his military service to the country. Pres Trump will follow established protocol for these situations and remain personally above equal pettiness against a political obstructionist to his presidency.
I believe that’s what he will do. I expect (imaginary) inside sources will tell the NYT that Trump went into a rage and condemned McCain to hades.