This is reverse logic surely. By this reckoning it's harder to play in the Championship for Swansea than it is for Manchester United in the Premier League.And he’s playing with much worse team-mates for Wales. Which should make it harder for him to make an impression. Swings and roundabouts. It’s obviously mainly a confidence thing. He believes in himself in a Wales shirt.
Wales are in a Championship/League One level group whilst being a lower PL side in quality so it's obviously easier to look good in that setting.
That said, it would be better to disregard international performances for all players when considering club performances. International football and club football are different countries, for want of a better phrase.