What is an external runtime in Vim?

I was reading up about setting up NeoVim and it is stated that NeoVim has an inbuilt Lua runtime, which makes it super fast.

Here is a reference to one of the posts regarding the same:https://www.reddit.com/r/neovim/comments/q7dp56/what_exactly_is_the_point_of_lua_in_neovim/

Does that mean Vim uses an external runtime? If so, what exactly is an external runtime?

If a runtime is a stage wherein the entity under concern is executed along with access to the supporting files required for its execution, I'm not sure how can there be an external/internal runtime and what would it mean for a text editor like vim/NeoVim.