That’s not what I said. We all make mistakes. The difference is that hopefully we try to get back on track.
I said once one slips OFF THE DERECH. By that I mean if one INTENTIONALLY STOPS KEEPING MITZVOS or in this case misreads the Torah so aveiros are seen as Mitzvos (or at best “neutral”). Since they don’t think their mehalech is wrong there’s nothing stopping them from applying it to more areas of the Torah hence the slippery slope, and they will bederech hateva never try to get back on track.
Of course there are always exceptions that can trigger Teshuva but this is my take on it.