Native promises do not have a "real" cancellation API (no 3rd path besides resolved and rejected) but you can mimic the functionality by rejecting with a dedicated error type, or use another promise as a cancellation token for example.
Aborting is specific to HTTP requests, it's stopping the connection to the remote server (so technically impossible to implement with the current APIs as you said). Cancelling a promise is just stopping all onResolved continuations to be called, so it can be implemented in several different ways as I explained above.
The end result if you use a classic cancellation wrapper on a fetch() promise is that the onReject continuations will be called immediately with a specific reason. The HTTP request will continue to the end (no abort), but the response will be ignored.
In the case of the react library I mentioned, both the onResolved and onReject continuations will be ignored if cancelled, it's the subtlety that allows the component to be garbage collected (and probably why the author decided to call that "thrashable" as opposed to "cancellable" to mark the difference).
For example, you can wrap a promise such that when it finally resolves, it checks an isCancelled flag and rejects instead so that your .then callbacks aren't run.
It's not about cancelling any potential inflight action within the promise itself.
In this case, you don't want the callback to run after the react component unmounts, nor do you want to hold a reference to the component:
