This sounds like it would be fine for code that you write yourself. But if you're only compiling code you wrote then C++ build systems are pretty trivial. The hard bit is dependencies.
One big lesson that newer languages like go and rust seemed to have learned is that the tooling, building and dependency management need to be dictated as part of the language ecosystem. Dealing with tons of other C++ projects written by other people (even in the same company) - how to specify dependencies, where their build artifacts can be found, etc - is a HUGE pain in the ass and consumer of my time.
They all get the tooling wrong though because none can stand the idea that you might want to mix languages, or add their new language to an existing project with existing tooling.
For Go, mixing languages is uncommon because its FFI suffers an impedance mismatch with its task scheduler. For Rust, mixing languages is extremely common. Rust's entire original reason for existing was to rewrite small parts of a large C++ project.
He's saying that Cargo doesn't really help at all with that mixing. Integrating Rust into a multi-language build system is indeed quite a pain. You have a few bad options:
* Drive everything with Cargo using `build.rs`. This sort of works but it's pretty horrible.
* Give up and just have your main build system run `cargo build`. This is what I normally do but again it's not ideal because it doesn't really integrate properly with your main build system.
* Give up on Cargo entirely. This is what Bazel does, and it's probably the most robust solution but it's still not ideal because most of the Rust ecosystem expects you to use Cargo (e.g. rust-analyzer).
I don't think any other languages have really solved this problem either but it is still an annoying problem.
Java, JavaScript, and Ruby didn't do this, and yet they all have solid build and dependency management stories. Java and JavaScript have even managed to have multiple build and dependency management tools existing at once without there being fragmentation and ruin. So clearly, having those tools dictated by the language is not essential.
I'm not sure why C and C++ have such a bad story here. Some combination of greater intrinsic complexity (separate headers, underspecified source-object relationships, architecture dependency, zillions of build flags, etc), a longer history of idiosyncratic libraries which people still need to use, the oppressive presence of distro package managers, and C programmers just being gluttons for punishment, probably.
Dependencies fit into this model, too. Presumably the dependencies build wherever they came from. Do that, package the output, and put it somewhere your project can use it in the suggested manner.