I think the "stick it to the Celtics" part is Woj's conjecture.
The article does a good job showing why Ray felt slighted, whether those reasons were real or exaggerated. Woj points out why Ray wasn't comfortable in Boston. I think he takes a logical leap, though, and assumes that he signed in Miami as a middle finger to Boston. I don't buy that. I think he chose Miami because they're going to be winning titles and he likes the city. Where else was he going to sign? Memphis (i.e., the violent crime capital of the country)? Minnesota (crappy organization, colder than heck)? Or Phoenix (would you want to play for Richard Sarver)?
He chose Miami because it was his best option besides Boston, and he didn't want to play for the Celtics any more.