That is partly correct. Actually, the North side of the Sierra Tower was the first tower and was called the Tower Annex Building. In 1965/66, they added the other half to the building. The Sierra Tower is actually two separately built structures joined together.
The Monorail station can give you perspective as to where the old, removed section of the Disneyland Hotel was. I remember calculating just where the Olympic size pool was and thought about all the fun I had in that pool. I also figured out where the Monorail cafe was.....too many memories and fun times there as well. When cruising out of the station on the Monorail, I used to look out at the Garden Structures on the Hotel grounds which were a favorite stay of ours when visiting Disneyland. Now I see a parking lot.