It should be fixed with Polonius, that's why it is mentioned in the paragraph after the example. Polonius reframes borrowing from lifetimes to places similarly to Ante so this is meant to show some advantages of this approach even without adding extra rules for unstable places, etc.

Ante shares parts of its approach with Valen - both having been based on the group borrowing concept. I'm in contact with its author and we share ideas every now and then. Ante is mentioned at the end of that article too. There are actually several upcoming languages using group borrowing (or something similar) as a base now - Ante, Valen, Carbon, Zeta, Fang, Mojo, and likely more.

There are a few parts of this overall approach that'd be difficult to fix in Rust without making it an entirely different language I think. The first would be separation of use vs reference, so users can pass around mutable references without the language thinking they may be mutated, and the second would be changing Rust's core semantics to allow shared mutability in general. With internal mutability like GhostCell this is allowed via UnsafeCell but I'd imagine making every reference UnsafeCell by default wouldn't be optimal and quite a few internal changes would be required.