In that talk he called C “the worst language” and said he chose it to troll the industry. How does that support your point?
He also said that you should choose “least privilege” whenever possible. That is precisely the value that Rust brings over C. So how does that talk support the idea that C is more secure than Rust?
Other than having to know Rust, adding Rust to a C code base is not difficult. They play well together.
There is no need to rewrite old code but, once Rust is there, you are free to.
Linux is a bit of a special case as you cannot just blindly use the Rust standard library.
Having to have a Rust tool chain to build with may or may not be an issue.
For some use cases, like BSD or the Linux kernel, platform support is also a consideration.