That is, WASM is a much more natural target for C++/Rust than anything higher level. It can also express unsafe code when you consider the issues brought up in the paper, i.e. that it's unaware of heap integrity. Conversely, the JVM/CLR was traditionally better for Java/C# like languages and you couldn't run C/C++ naturally.
This makes sense as I've heard some of the more recent C# features are to recover performance, like value types, slicing, etc.