> Why didn't they go with `defer` which is the more ergonomic and established pattern?

Neither statement is true.

> `defer` also doesn't need to change the API of thousands of objects to use it

Callbacks can trivially be bridged to `using`:

    using _cb = {[Symbol.dispose]: yourCallbackHere};

There is also built-in support for cleanup callbacks via the proposal’s DisposableStack object.

> or for things that are not objects

This is javascript. Everything of note is an object. And again, callbacks can trivially be bridged to using