I think they are. Rust is a general purpose programming language, and Lua is a language specifically designed to be embeddable. But that means if you're making a project that embeds Lua, like Factorio for example, you have the choice of what language you write the code in. The choice is whether to write a function as native or to write it in Lua, and that choice might not be super clear unless someone else already made that choice for you.

This is completely different than assembly vs PHP. Assembly is a 'don't use it unless you have to' programming language, while PHP is a programming language designed for website building. You wouldn't use assembly to make a website because there are no frameworks for it and you'll scream and cry. You wouldn't use PHP to write the entrypoint to your operating system because PHP can't do that.

Rust and Lua are a lot closer together than PHP and assembly and it often would be reasonable to compare these languages for the same problems. It's not relevant at all to the blog post though because the engine they're using is built for Lua, and it's not really up to them unless they want to work against the engine and try to use a different language.