Because of the certainty and clarity expressing the object was moving in line with his observation point, making no attempt to assert whether it was coming toward him or moving away from his location, I’d say that increases is credibility.
Yes, Mr McKay's email is refreshingly detailed and free of BS and incompetence. Of course, that doesn't mean it actually was MH370 that he saw. Unless the airplane stayed aloft and turned towards him after losing its transponder, the distance would seem to be a little far (365 miles from the point of last contact to Mr McKay's location). From how far on the surface should you be able to see a plane fire at altitude?
The mods deleted my posting of his email, but you can read it on Twitter here (click the image for a legible view).