I think Theo's reply was a bit excessive in some regards but I can also understand him being frustrated when you've been working on a very complex project for decades and somebody comes around and say "uh, why don't you rewrite it in <hype new language>?".
It's seldom a very interesting or constructive discussion. If you think Rust has something to bring to some project then you should at least take the time to write a decent proof of concept or something similar, not ask the current maintainers to rewrite their codebase in Rust/Go/Haskell/Prolog because of some purely theoretical benefits. You don't go to construction workers in the street and tell them that they should consider using different types of hammers.