Not sure what this is. The homepage shows making an API in Lua. So is a web framework?

In any case, a bad description term-wise. Being referred to as runtime made sense for Node & JS, since JS was until Node mostly confined to web browsers with Node setting it free giving native OS access, an event loop, and even a module system. Lua, Python, etc are already shipping as their own self-contained runtime. So calling a Lua, Python, etc wrapper in X as runtime written in X makes no sense.

The "runtime" word is very confusing in this case.

It's a web application server written in Rust (Axum/Tokio), that has hooks into Lua bindings (mlua) so that application behaviors can be implemented in Lua.