I don't think you can say that for certain. He may have been able to play like this a year ago, but we will never know as he was getting so little playing time. Sancho left and got more playing time and look what he achieved while Foden has been mostly on the bench. And yet most on here think Foden is a better player including myself.
Midfielders and wingers develop at different rates. It's much more difficult for 19-20 y.o. players to shine in midfield than on the wings.
Where would one loan Foden out to help his development? Championship would be probably stupid and no PL team remotely resembling our style would guarantee him a starting place. Learning under Pep and with the Silvas, De Bruyne, Kun, Raz, Kompany etc might prove much more beneficial in the long run than quickly becoming a star.
People often fail to understand that there might be important tradeoffs between quickly establishing oneself and having a higher ceiling and hitting greater heights. Foden will surpass Sancho, imo, precisely because he was patient enough to learn under the best manager and with the best midfielders.