That's partially the joke. The other part is in the preceding sentences:
> C++20 brings us modules, an innovative new feature, and one I’m looking forward to actually being implemented fairly soon. Looking forward, C++26 will likely have stackful coroutines, the ability to embed binary file contents to initialize arrays, a safe range-based for loop, and many other goodies.
> By comparison, the pace of innovation in Rust has become more sedate.
The reason C++ is getting all these new features while Rust isn't is not Rust being held back by backwards compatibility, but that Rust has all those features already.
So he's basically saying: C++ is breaking backwards comparability to (eventually, years after the spec was published) bring us features that Rust already has.
> C++20 brings us modules, an innovative new feature, and one I’m looking forward to actually being implemented fairly soon. Looking forward, C++26 will likely have stackful coroutines, the ability to embed binary file contents to initialize arrays, a safe range-based for loop, and many other goodies.
> By comparison, the pace of innovation in Rust has become more sedate.
The reason C++ is getting all these new features while Rust isn't is not Rust being held back by backwards compatibility, but that Rust has all those features already.
So he's basically saying: C++ is breaking backwards comparability to (eventually, years after the spec was published) bring us features that Rust already has.