Since Mathis is vetted true (via MI-6 torture and interrogation), Le Chiffre said this to Bond, who trusted Vesper wrongly (that’s why watching CR is fun when you know Vesper is Vesper, you can see why doesn’t want to give the money for the buy-in to Bond, she does not want him to win the poker game because she is growing to care for him, etc.) to confirm Bond’s incorrect suspicion that Mathis, not Vesper, mentioned the poker tell.
In the torture room, Le Chiffre’s henchmen fake-abuse Vesper, who screams fake screams of agony, to put further pressure on Bond to give up the account with White’s/Quantum’s owed money.
In point of fact, no one may have told Le Chiffre about the tell, he may have noticed his own tell in his stellar poker career, and set Bond up for the poker loss personally. What Le Chiffre certainly knew is that Vesper had a call from Mathis (or lied to Bond to say she had a call from Mathis–“meet me outside the lobby”).
It is a matter of humor that under stress, Le Chiffre later slips into his old tell and folds a hand.
Mathis is good, but definitely dead, and that’s how it goes. “From what I understand, Section Heads have a short lifespan, at least for Montenegro and Prague…”