Things are moving on from the dark days and people/communities are reconciling. Certainly that is true. However, I think it is important to understand the context of the gesture so that we can actually try and interpret/discuss it properly. The gesture is ultimately rooted, not in bitterness or hate, but in a self-respecting fear of appearing subservient. Niall McGinn explained this on behalf of those nationalist lads who do the same thing when lining out for NI. James evidently doesn't hate the English or British; he gets along fine with them every day of living here. Likewise, he has no issues with his team-mates from NI and gets on perfectly well with them.
This fear or insecurity of appearing subservient, however, is very naturally more pronounced in a society moving on from a conflict that involved two "mutually exclusive" or incompatible sides attempting to violently out-balance, domineer and subjugate one another. For nationalists in the north, 'GSTQ' remains a very real symbol of those who, not just in living memory, but also contemporarily, gladly lord it over you and who crudely and insularly see your falling one step behind as a progressive leap for them. They measure their success by your failure. If you're a nationalist who is secure enough to be able to overlook that, great, but I will defend the likes of McClean if stepping above that is something over which he still has strong emotional and logical reservations. I won't hold him to blame for a reluctance to psychologically/apparently subjugate himself to an ideology of supremacy. Why should be feel bad for it?
Eamonn McCann's critical analysis of Saville is well worth a listen, by the way, if you're interested:
https://www.youtube.com/watch?v=x83lt5zDRbg