I think it's a cultural issue, but I honestly don't see what's so offensive about this comment (I was expecting something far worse), especially if it's factual assumptions are true. Honest question, can somebody explain the issue here to me, and how one would express these concerns otherwise? I feel as if I just don't get American culture of politeness and niceness to appreciate it.
It's a personal attack. The poster is telling the maintainer to quit writing Rust, implying he can never get better and basically demeaning his whole effort.
Do you not see how that is (a) not related to the project and (b) rude in any context?
If you were, say, singing karaoke and someone came up to you and said "you're awful, never sing again, you're off-key, you don't know the words"... Is that not rude to you?
I don't claim to know much about either karaoke or rust, but from what little I know of both, that seems wrong. The Rust community expects safe code, while bad singing is expected at karaoke bars.
The comparison was about how to communicate respectfully and appropriately, not about making an perfect analogy.
Think of it this way then: it is completely possible for the Rust community to expect safe code without being entitled assholes about how they communicate that expectation.