What did he say? I can't find his comments.
I don't think he is right though.
“It overshadowed things. It killed that England team, that generation. One year we would have been fighting Liverpool to win the league, another year it would be Chelsea. I considered the Premier League to be my title. So I was never going to walk into the England dressing room or the canteen and open up to Frank, to Ashley Cole, John, Joe Cole at Chelsea, or Steven and Jamie Carragher at Liverpool. I wouldn’t open up because of the fear that they would then take something back to their club and use it against us, to make them better than us. I didn’t want to engage with them in that sense.” - Rio
Perhaps if he had done it in his time, England would have been more successful?
I think it being beneficial depends on the person. To some it'll be a testament to team spirit, and to others it'll be a superfluous gesture. I think something that needs to be accounted for is that the Welsh players have spoken about how much they enjoy playing for the national team, and this can only be advantageous. That famous England side (the golden generation) that had much more talent, were pretty outspoken about how they didn't really enjoy playing for their national team, they mentioned that the domestic club football was more important, and their were rifts between the players depending on which club they played for. I've quoted Ferdinand above, but there are similar comments from the majority of the players who played at that time.
Wales getting to the semi finals in 2016 was a gargantuan effort, and beating Belgium was a genuinely brilliant achievement. Without the unity that Wales showed it would never have happened, the same way that most people thought Wales wouldn't win a game and would be bottom of their group, and yet got second.