alt Hacker NewsApologies for not understanding. And I admit, it's hard to communicate in the abstract, so I might still not understand the question.
If it helps: AFAICT, Valen's borrow checker preserves the same ecosystem-wide concurrency benefits that Rust has. For precedent, check out GhostCell [0] which is not only _compatible_ with Rust's concurrency but gives it some interesting new abilities. Valen's approach could be thought of as a more ergonomic form of GhostCell that better tracks the relationships between multiple groups (brands).
I could give a better answer if we had an example to toss around where we think Valen might force things to be single-threaded.
Rust needs shared xor mutable in order to prevent data races on non-atomically accessed data. You said that Valen can _express_ shared xor mutable, but perhaps you haven't clarified whether this also interacts with equivalents to Rust's Send and Sync to enable the same fearless concurrency.