"Someone told me my idea sucked" is not in and of itself any more of a useful signal than "someone told me my idea is brilliant"; one person's opinion is, well, one person's opinion. Who that person is makes a difference. In the case of SigmaOS here, the "(YC S21)" suggests that they've had at least one very positive signal: YCombinator accepted them. "The most famous seed funding group in the tech world likes us enough to work with us" is probably more of a meaningful signal than "SnarkyTechGuy1337 left us a cutting comment on Hacker News".