By that logic, how is he not a Welsh legend? He's probably their best ever player, maybe worst case top 3. Other contenders are Charles, Bale, Southall, Meredith, Rush, and Sparky. Maybe only Charles and Bale are rated higher?
If he qualifies as a United legend then he should also be a Welsh legend. You either don't have him as a legend in anything due to his off-field issues or have him as one in everything despite him being a horrible cnut.
He skipped so many Wales games in order to gove his best to Man United (and then eventually quit altogether) , that's why many don't consider him a Wales legend.
For someone who played for 24 years, he only got about 60 caps. Obviously that helped him have such an incredible club career.