When Thrawn reads Ahsoka's bio he gets a curious, knowing look on his face and says "So, Ahsoka Tano was the apprentice of General Anakin Skywalker", because Thrawn once knew Anakin Skywalker. This is a bit of a nod to Timothy Zahn's prequel novels about Thrawn. The novel "Chaos Rising" takes place in 19 BBY near the end of the Clone Wars, at the time Thrawn was still an officer in the Chiss Expansionary Defense Fleet, the Chiss were facing a threat from a new enemy. Thrawn left Chiss space in an effort to search for allies and/or new technology to fight this enemy, his search took him to a planet near the edge of the Unknown Regions. This planet also happened to be the location of a secret Separatist base protected by a planetary shield generator, while at the planet Thrawn encountered Anakin Skywalker on a mission to destroy the base. Thrawn agreed to help Skywalker with his mission in exchange for allowing him to take the shield generator as the Chiss did not have such technology. Later on in the novel "Alliances", which takes place in 2 BBY after Thrawn joined the Empire and obtained the rank of Grand Admiral, Emperor Palpatine sent both Thrawn and Vader on a secret mission to investigate a disturbance in the Force that Palpatine felt. This was Thrawn's first time working with Vader, their mission took them to the same planet that Thrawn met Anakin at, and thanks to Thrawn's exceptional powers of observation and deduction it was not long before Thrawn was able to figure out that Darth Vader used to be Anakin Skywalker, making Thrawn one of the very few people at the time that knew Vader's secret. Hence Thrawn's reaction to learning Skywalker was Ahsoka's master.