A thought provocation:
Did they actually believe in Tahreef or are they deceivers that state what they don't actually believe themselves?
Before dismissing the latter as some sort of loon conspiracy - see what happened to Bani-Israel regarding their Messengers, and see what happened to the teachings of Jesus. And see what Quran has emphasized are the primary enemies of the Prophets.
It's time for humanity to wake up. There are some questions that would follow:
1. How do we trust the narrations they narrate then.
My answer is we don't, but we don't dismiss them all not due to who narrated them, but due to the truth they contain, the truth that fabricators themselves were oblivious to despite narrating these truths.
Just like the bible contains teachings regarding the face of God that prove the proper religion but the ones who mixed with it falsehood don't perceive these teaching.
2. How do we determine the truth?
And this is what all revelations came to teach humanity and Quran has an elaborate explanation on all the steps humans must take to accomplish this.
And it cannot be a go it alone approach but needs believers to counsel one another regarding their affairs in following the foremost of those who race ahead, obeying those who hold the Authority, asking the family of the reminder and refering all disputes among themselves back to Quran and Sunnah.
And how to counsel one another has been explained, and dictating and trying to get people to center around you and follow you as if you are an authority and obey you is forbidden for believers, that is the position that belongs to the chosen ones.