I write Rust and C++ for a living and completely agree with this. While I do like Rust in general, the language is so complicated that anything other than trivial programs is going to take way longer to implement than in any other language. Testing code in Rust is also half baked at best, I find that any other popular language has better capabilities than Rust in this aspect, including C++. I'd even go as far as saying that for a startup I'd use C++ before Rust if performance was a feature, otherwise I'd just go with something that has batteries included like Python or Golang.
Depends on what you're doing. There are apps that can be written in Rust much more quickly than in other services, cause you don't have to worry about a lot of stuff (like for example it's hard to keep an open connection for every client and process everything real time in Ruby on Rails).
It will take 2x more time to develop same thing than using something like TypeScript/Python and you will have much smaller and more expensive talent pool. Startups usually have to make, at least minimum viable, product quickly and start selling it before funds dry out. That means using familiar tech with big ecosystem.
Also choice of tech is probably least important thing contributing to startup success. That knows anyone who worked in terrible codebases that were generating multi-million revenue. Customers don't care about your tech. Simple as.