They definitely have opened some things up, like a full C++ interface to Metal. I think the idea with this current effort is kind of the opposite though - not to encourage more use of C++, but to start writing new features of C++ projects in Swift (the presenter at the C++ Now 2023 talk [1] goes into this). I expect WebKit is a big target for them, I think they eventually want to replace all their C++ with Swift gradually (instead of trying to do ground-up rewrites, where you dedicate a team to doing that for years and you have a replacement at the end with fewer features than the old project).
1. https://www.youtube.com/watch?v=lgivCGdmFrw