The difference is a that a developer has a lot more control over what version of Deno their server-side app runs on than they do what version of a browser their client-side app runs on.

This isn't really true in the wider ecosystem. Yes, the app developer does, but the library developer does not. As Deno adopts newer versions of TypeScript and config options libraries can fail to compile. This is compounded by the lack of a package manager so that references to specific versions and CDNs are hardcoded into dependents.

All with essentially no benefit over running plain JS with associated typings.

They don't though. They transpile it behind the scenes

Same thing with the same problems. The difference being that Deno isn't going to have the same no-breaking-changes policy that the web does.