Samper isn't a carbon copy of Xavi, who had the biggest match-impact of any footballer I've ever seen, but still a deep lying playmaker. He loves to pass and look for space, playing a big role in transitioning the ball forward from deeper areas. Naturally, that means his game relies on the movement/abilities of those around him. There's no way he can look great when the midfielders he's playing next to are the likes of Bicho, Gumbau, Patric, Munir, Halilovic, etc.
A lot of Barca B games are composed of us having a hardcore struggle to bring the ball out of defensive areas because of the center midfielders being utter shit at making themselves available or being useful at all in buildup. IF we do bring the ball forward, it's either down the wings or scrappy long-balls. The other portion of matches is spent pinned back in our own half due to the pressure of the opponent and the tendency of our players to try to mount incredibly weak, rushed counterattacks that just lose possession again (refer to the Valladolid match as the most prominent example of this). Together, that leaves an incredibly small fraction of meaningful play for Samper to show his skills in buildup.
Good team performances are the environment that more accurately show what Samper is capable of and are a much more accurate representation of the kind of matches he'd face at top level than a chaotic relegation battle playing with B team scrubs. When he played with the first team against APOEL he was the best midfielder of the trio, and he stood out in other first team appearances as well. He showed a level of maturity at the highest level that absolutely none of his reserve teammates did other than *arguably* Sandro. He might not have been imperious with the B team but it's very hard to believe that any of Halilovic/Adama/Munir would perform better with the first team than he would.
Lucho's changed his opinion on plenty of things. He moved Messi from CF to RW when he saw that'd be the best place for him in the team dynamic. For a more relatable decision, he moved Roberto to DM because he saw that he's better with more space/time and has the potential to grow defensively. Samper will, by default, have more first team chances this season than last (his growing older, Xavi leaving, etc.) and that can be extended further if he impresses Lucho in preseason. I'm willing to believe that Lucho will strongly consider him at more than just DM when Roberto is now seen as the de-facto rotator for Busi. If it's somewhat of a priority to give Samper minutes to develop but Roberto is occupying the DM position, Lucho can easily push him forward to accommodate. The list of reasons to push him forward is growing and I don't think Lucho will ignore it forever.
Moreover, on the point of Samper being Pirlo-esque rather than Xavi-esque I don't see how that is a negative. He might not play with the same type of authority as Xavi does but with Busquets' new responsibility and Rakitic, I don't consider that an issue. A 'Pirlo' type player still would have a large part in building plays and helping the team to control space even if he's not the one orchestrating everything. I'd honestly love to have a player of that profile successfully fit into our midfield since it's what I feel we need more of (someone who can pass between the lines from deeper areas to start attacks).
It's not meant to prove that he's had a great season with the B team, because that'd be untrue. It just shows that his level and talent is still very much there even though the team he's playing in doesn't allow him to show it on most occasions.