The Matsui scene.

Lyrics from the Led Zeppelin song "Kashmir".

To be honest, there's a significant number of clues that it's just an elaborate prank on Linus even before the confirmation by his mother near the end. I mean, think about it: Linus approaches Rusty saying he wants to be brought along to the negotiation, Rusty explicitly tells Linus that he's not ready (and the implication is that neither he nor Danny want Linus at the negotiation at all) but Linus doesn't let it go and Rusty eventually says he can come and that he'll be available if Linus has any questions. Once they get to Europe, Rusty purposefully is not available despite Linus repeatedly trying to contact him and, on the way to meet Matsui, he and Danny purposefully overload Linus with contradictory instructions (similarly to how he did back in Ocean's Eleven when Linus was preparing for his role in the heist) which puts Linus on the wrong footing before the meeting even starts, and the two of them refuse to give Linus any help whenever Matsui puts him on the spot (if the negotiation was at all real at that point, they would have engineered it so that Linus wouldn't be able to even open his mouth in case he potentially ruined things in the way they all pretended he did) and wait until he gives them an "excuse" to hold the meeting without him...which is exactly what they wanted in the first place.

Basically, they were trying to teach Linus a lesson, that he's still effectively the rookie in the group and still has a lot to learn, which helps further Linus' arc not only in the context of the trilogy as a whole but also just within the context of this movie as he goes from being still fairly unsure of himself (he hesitates when Rusty asks if he truly thinks he's ready for a more central role, after all) as he was in the last movie to actually coming to believe (and prove) that he's capable of operating in the same league as Danny and Rusty by effectively stepping up to become the leader once the others have been arrested.