His priority should be to the organisers of the tournament he is playing in at the time.
Yes, I agree. I'm not saying his thinking was right, just that that may have factored into his decision. I also agree that leaving it as late as he did was inconsiderate and really left the organisers with a dilemma. But as far as his back is concerned I've yet to hear a plausible reason as to why he would withdraw if he wasn't injured. His argument with Stan? Perhaps...
Anyway, with regards to Andy's tweet, I'm inclined to believe that his clarification was what he had meant all along. I don't think he was having a go, or trying to be "funny", he was just making an observation. Some have said that he was implying that Roger was making up his injury, while others have said that he was implying that had he played Roger a couple of days later he might have beat him. I say both of those inferences are absurd and everyone should #relax.
And by "everyone" I pretty much mean angry Federer fans.