Pointing out shortcomings isn't impolite - writing that Go wasn't designed, but happened by accident is however (IMHO).
That's one of the most mild and inoffensive criticisms I've ever read on the internet. You can't seriously be indignant about that.
It's also more accurate to say that Rust wasn't designed / was an accident since it was technically a side project of one guy at Mozilla.
One can plainly be a good programmer and yet create a bad language. I'm sure you are well-informed enough to analyse his argument on its merits, even if it were written by a gang of monkeys with typewriters.