I know that he can play there but the reason they can sell him is because it's not ideal, he said it himself.
I think there's many reasons why they are selling him
A- Barcelona are spoiled for choice in CM. They have Gavi, Nico and Pedri who are awesome while they look set to sign Kessie as well. Kessie and Gavi can't be signed (or given contract renewals) unless they get rid of De Jong.
B- Barcelona need money and De Jong will be on a monstrous salary next season
C- If Barcelona tried to sell one of their homegrown gems then the fans would burn the stadium. It would be like us selling Scholes when he was in his mid twenties to keep Veron. It ain't happening. The only midfielder Barca's board can sell for good money without losing their heads in the process is De Jong
D- De Jong had been neat but not exceptional. That is down to many factors including Xavi not being able to use him correctly and him being a former homegrown talent himself. If lets say we had Scholes as manager then we're more likely to play and keep an on form Rashford then an on form Martial simply because the former is a homegrown talent. Homegrown talent tend to stick together.