The same people who were working on generics in Go prior to v1 were also still there to help out when it eventually landed, so whatever stones, clubs, and pitchforks you saw weren't compelling enough to scare one away from making it happen. Such things don't get in the way when do something is worthwhile.
But obviously "go doesn't have generics lol" comments were even more important. Imagine what would have happened to HN if that wasn't repeated multiple times daily for years!
To offer help, the receiving end must be open to take it.
And there was plenty of help given by others who also weren't experts. There was no lack of trying within a sea of incapable people. Unfortunately, that wasn't what the project needed. It already had a team full of blub developers, it being a language for them.
The Haskell muscle had proven their skills already with Featherweight Java, in 2006.