You're right. There is no passing of time on the Astral Plane, so Githyanki have the leisure to learn lots of stuff.
In Mordenkainen Presents Monsters of the Multiverse, Githyanki are described as having a Trait called Astral Knowledge. In the book, this lets them connect to the Astral Plane when Long Resting, access "reservoirs of experience", and choose one Skill to be proficient with until the next Long Rest. In BG3, they get one Skill Proficiency for good.
