Dear Goatmilk,
If YOU had paid attention you would have realized that the Joni Mitchell "Blue" album showed up when William's sister left. Remember? She hid the albums under his bed. That WAS in 1969.
On the "special" DVD version of this Cameron Crowe does commentary and points out this album during the scene. His comment is (paraphrased): "I know this album wasn't out at this time, but it was so influential to me that I wanted to include it."