I absolutely disagree that he is restricted as a CDM and "unleashed" as a CM. In fact, at Barca I think quite the opposite is true especially under Valverde.
To say Frenkie plays like a Xavi role is somewhat lazy and trying to fit him 1:1 into something he isn't. They are both excellent at press resistance, ball progression, and dictating play, however they do it in different ways. Xavi plays more as an 'outlet' player, receiving the ball in traffic and higher areas in the pitch. He dropped back a lot, but a large part of Xavi's behaviors on the pitch came from him existing in those higher pockets of space under pressure. From there he'd receive passes from any direction, swivel around, and distribute it into any other direction.
FDJ tends to start plays from deeper positions, facing the opposition goal. He tends to drive forward more aggressively/vertically from deep positions and that's how he initiates a lot of his linkup and passing. His best plays tend to come from him "building up a head of steam" as he either dribbles or 1-2's his way up the pitch in either direction. That's momentum on the ball, and it's something he needs to do his work well. He also wins a LOT of duels/interceptions like that (starting deep and pushing up to catch them off guard) and did so against Napoli to assist a goal. He is good as an outlet because he has excellent awareness but he isn't quite as agile/technical as Xavi in the tightest of spaces and doesn't tend to do his best work in pockets like an outlet player.
Just look at his best performance against Napoli. He was used as a CDM/Regista, and played with TOTAL freedom and dictated the play wherever it was on the pitch. Since FDJ was often the first player making a run/pass out of the back, that gives him the power to dictate the quality of the entire attacking move from the very beginning. He broke the initial opposition press with dribbles or splitting passes, then he made followup runs and plays in support and also develops the attacking move further until its conclusion. From the CDM position he had total field vision of everything in front of him and made key runs forward (on either side, left or right) to set an aggressive offensive tempo.
On the other hand, we saw him play as LCM with Dembele for like 30 mins in one game. That was a pretty good performance and FDJ did make nice plays at LCM combining with Dembele, however he had fewer touches on the ball than he did as a pivot. We saw it again when Arthur/Coutinho came on in the next game and there was some unspoken positional switch and FDJ switched to the interior while Arthur was the pivot. FDJ had FAR fewer touches at LCM and our poor organization often left him isolated, with Coutinho totally abandoning the LW.
FDJ clearly had way more impact on the overall game and the level of our offensive play when deployed as a Regista style CDM. We need to stop this restrictive "Xavi role" or "Busquets role" type shackles. FDJ plays in the position of Busi but his style of play gives him freedom to roam offensively like Xavi does.