I don't think the post was trying to prove that fact, just explaining a personal choice. I don't think every Rust project should have to include benchmarks to prove that Rust was the right choice.
Anyway, the Playdate community has done some testing and the Swift code for the Life example, as released in the blog post, runs about 60% the speed of the C example it was based on.
So it seems the claim was an assumption that it must be performant because it's Swift, rather than the result of any comparison or testing. It looks like I was too optimistic :)