I think you are talking past each others. Preventing N+1 by doing lazy fetching and having synchronous / asynchronous API are orthogonal issues. Async API must not block the thread/event loop when the data loading is being done.
The creator said it is production ready, and that it will never be "officially" part of diesel as diesel only supports a sync interface. Nevertheless, diesel_async is fully fine to use, he mentions.
Diesel hasn't been providing an async API for reason told in this thread: https://github.com/diesel-rs/diesel/issues/399
The situation might change some day though, once async support in the core language and surrounding ecosystem gets stronger.