That "saying" does make sense.People who say true believers can never fall away read it as saved people never fall away unless they were never really true believers to begin with.
Folks can fake real belief to impress others for only so long.
With 1 John 3:9 in mind, and a belief that an apple seed cannot bring forth onions...or any other thing but the fruit of the apple seed, I see no way for God's seed to return to bringing forth the devil's fruit.
That is the difference between "belief" and "true belief".But I'm with you. I see it saying you're saved as long as you keep believing what you heard in the beginning. John says the same thing.
1 John 2:24
24As for you, let what you have heard from the beginning remain in you. If it does, you will also remain in the Son and in the Father.
That's why I say the whole thing boils down to whether or not a true believer can ever stop believing. Reformed theology says they always will, or else they were never true believers to begin with. Non-reformed theology says you remain a saved person in Christ as long as what you heard in the beginning remains in you.