For example, the server can register a DOM listener closure as part of SSR, and then only materialize (download and resume) the closure if the user actually triggers that event.
Serializing closures require that both the bundler and runtime cooperate. In all existing FWs, the FW runtime does not care about bundling. Hence they can't serialize closure, and the FW can't be resumable.
Adding bundling information to runtime after the fact is extremely difficult, which is why I predict that it is unlikely for any FW to retroactively gain true resumability.