is the foundation of every other sin there is.
He wasn't only referring to the fact that pride was the cause of Satan's fall and thus the foundation of every sin ever to occur subsequently, but he believes that in each of us as individuals, a part of each sin we commit (be it adultery, gluttony, murder, whatever) is pride.
What are your thoughts on this?
He is partially right.
There are many sins caused by other devices, like loathe.
Suicide, which is a geat sin, is rarely caused by pride. Most often in the western world it is due to loathe.
Gluttony also can be caused by an etreme low value of self worth.
Most cases of theft are not from pride, but from loathe, like feeding a junky feeding drug habit. Drug addicts often hate themselves and view self medication as the only way out of their addiction, there is little pride there.
In my opinion, Sins are caused by people out of balance. I think that if you are centered you won't have a reason to commit sin.
In my view
Loathe = Despair in hope—Those who despair in hope, cease to hope in salvation from God or help in attaining forgiveness of sin (CCC 2090). Christian hope sustains a believer’s faith and dependence on God, and should not be neglected or rejected.