Are they trying to catch up to Rust while maintaining backwards compatibility? Or what?

C++ needs the full definition of a class, including its private members, before you can use its methods. This is a huge dependency headache. Most later languages have interfaces or traits or something to decouple class definition and interface definition. This looks like an attempt to deal with that. Of course, this is a retrofit, so it's going to be messy.

C++ has a lot of baggage. It may have exceeded its excess baggage allowance. The basic problem with C++ is that it has heavy encapsulation without memory safety. No other language has that. Either nothing is hidden, as with C, or the language is memory-safe, as is almost everything since C++.