I suspect part of this is that there's prescriptivist nitpicking and non-prescriptivist, uh, non-nitpicking.
Pointing out that "feedback" in uncountable is giving information on how to better match actual usage. A prescriptivist will agree with it, but so would a descriptivist.
Contrast this with "lion's share": traditionally, it meant "all", but nowadays is usually used to mean "most, but not all". If I convince someone that they should use it to mean "all", they'll be further from matching actual usage. I think that's what qualifies it as nitpicky and rude vs. just helpful.
Pointing out that "feedback" in uncountable is giving information on how to better match actual usage. A prescriptivist will agree with it, but so would a descriptivist.
Contrast this with "lion's share": traditionally, it meant "all", but nowadays is usually used to mean "most, but not all". If I convince someone that they should use it to mean "all", they'll be further from matching actual usage. I think that's what qualifies it as nitpicky and rude vs. just helpful.