>They tried to submit a paper to Nature, but was rejected
This should be "were rejected" because "were" refers to "they". It cannot refer to the paper even though that would make sense. You could say "but it was rejected" to force the sentence to refer to the singular paper.
Okay, if we don't correct English on HN that's okay. My foreign friends ask me to do this with them but I get that HN is a different forum and that we are not friends.
There was nothing offensive or corrosive about the way you stated the correction so I don't see why you should be down voted; as someone who is learning a foreign language, I can say that having this kind of correction is useful.
Rewarding the mob for its hysteria and stupidity by hiding away from them is the wrong move in my humble opinion; after all, you're only losing internet points which aren't real.
This should be "were rejected" because "were" refers to "they". It cannot refer to the paper even though that would make sense. You could say "but it was rejected" to force the sentence to refer to the singular paper.