“they have at least convinced themselves of it”
The fact that there are some who do is not the discussion here. The OP asked why is it diff, and the answer waw given. Asides that you are incorrect, there are many Talmidim who came to the Rav or Rav Berel and they completely changed their derech halimud, and they were mekabel as rav haMuvhak. However, the fact that many don’t shouldn’t be any diff of what it say. אחת מאלף יצא…