The difference is that nothing what eyko said is based on hearsay.

You can, but I'm not that anonymous (one can easily find my name by searching for my username, not that there's much interesting to find anyway). More importantly, I wouldn't try to damage a person's reputation if I wasn't ready to stand behind what I said and, if proven wrong myself, take some responsibility. I think a few negative things were said about his personality, which can be damaging. An anonymous internet comment is not the most credible source though.

Disclaimer: I don't work for Meta, don't know Carmack, etc.