Disclaimer: one of Quinn's maintainers.

Actually, Quinn is split up in two crates: quinn is the tokio-based async layer, but quinn-proto provides an async-agnostic state machine (and provides a trait-based abstraction over its use of rustls and ring).

Also, I think quiche and neqo are squarely aimed at serving their particular corporate goals, whereas quinn is really aimed at being a library for use in the Rust ecosystem. So far that has mostly been the tokio ecosystem, but that might change as the async/await story matures.