You have quite literally linked the proof yourself, and you know it. If you struggle with English, that’s not really anyone else’s problem.
90% of received feedback is negative. The only reason to mention that ‘they can’t know what other people feel’ is to ascertain that their opinions would be positive. Nothing else makes sense. If 90% of received feedback is negative, yet you do nothing about the issue, and then say something like that, you are claiming that ‘people who don’t give feedback would be positive’.
It’s quite simple, really.