He's 3 years older than Sancho and has 100 apps for Barcelona in La Liga, a much better league than the Bundesliga.
He's not better as a b2b just because he does better in a double pivot than as the lone no 6. In fact his best spell at Barca was under Koeman as the DM when Busquets wasn't available or as a CB even. He also plays as the sole no 6 for Holland, which LVG considers his best position.
Him having to play b2b at Barca is one of the reasons he's been relatively underwhelming. He's a deep lying playmaker.
The reason he does better in a 2 is not because he cannot defend as some have suggested. It's because he can be more adventurous (ETH's words) on the ball and take more risk.