You are right the Holy Spirit never has and never will make a mistake .Holy Spirit makes no mistakes. People do. Two points about that.
1. The word doesn't say that these people are not salved people. Falling away from faith could mean they are prodigals, for which God does not cast away.
2. As I said, there a re all sorts of people who claim faith among us but have not had saving faith. These are those who easily depart.
People can have the appearance of faith, but have nothing. They are dead, spiritually.
The Holy Spirit said through Paul that some will depart from the faith .He did not say they did not have saveing faith or they must not have had real faith or they never truly had faith or may have been raised in a church but did not really have faith ,He said they will depart from THE FAITH .
Why would anyone try to change what the Holy Spirit says. When He says THE FAITH I have faith that He means THE FAITH and not something else .
If He had said from ( A ) faith I could see someone asking which faith or what kind of faith .