A C++ lambda (as essentially is the case for a Rust lambda as well) is just an unnameable class object with an overloaded call operator, which makes the closure function body itself just a regular member function ABI-wise. You're not calling these things outside of the language, because they're only really callable via the language-specific static dispatch mechanism of templates. If you need to be able to dynamically call a lambda, you're going to be stuffing them in an object that contains a pointer to the lambda object and a pointer to the function of the actual body, something that looks essentially like what GP's assertion of what a lambda ABI looks like.

You can stuff it into something such as std::function_ref which erases the Voldemort type and then it can be dynamically called without needing templates. And this could be from C or other languages if only there was a common type in C we could use.