What is more, the Clone Wars Era events of Thrawn: Alliances took place months after Ahsoka left the Jedi Order, and thus Anakin wouldn't have had any reason to bring up the fact that he even had a Padawan when he and Thrawn met for the first time.
Thrawn in Alliances learned Anakin used to have a Padawan from Padmé although she only mentioned Ahsoka by name once (and only by her first name), and the sentence where Padmé said her name does not directly confirm she was the Padawan in question.
What is more interesting is that Thrawn learned of the Marg Sabl maneuver that he would use in Thrawn: Treason from this conversation but did not learn of the complete history of the maneuver or (as far as he could have interpreted from Padmé's wording) the name of the person who invented it. Thrawn probably put two and two together at that moment and realized that he was familiar with and had indeed used Ahsoka's tactical thinking after all, making him a whole lot more familiar with her ways than he realized at first.