Honestly, having seen the two of them together a few times briefly, I believe their love was real. Arthur stood by Tom's recovery from alcoholism and supported Tom's then-crazy-seeming idea to subdivide their property in Quogue (which Arthur bought with his share of the film sale of West Side Story) and build and sell vacation homes. Arthur was very proud that Tom made "millions" in real estate.
I'm sure Tom DID make a deathbed request that Arthur forgive Patti and direct her in Gypsy on Broadway. And I'm glad he did.
That said, I think Arthur is completely manufacturing the story about Tom and the Bogota production of West Side, because he's found out that he gets sympathetic press whenever he mentions his dead lover.
It's the kind of cheap sentimental trick that an Arthur Laurents character would think of--and get away with.
There, as Arthur lies to say, you have it. Hard as it is to take, it's the truth, so accept it and move on.