They don't have exercise for some of the more advanced college level topics. You can see the exercises on the right side of the screen under "practice" for some of the more introductory topics:
https://www.khanacademy.org/math/linear-algebra/vectors-and-...
I just tried clicking on some linear algebra topic, but it seems there are just videos
https://www.khanacademy.org/math/linear-algebra/alternate-ba...