Sure this makes the problem of mocking the server less painful. Well done. But I'd take completely integrated tests over these any day. Sure they're slower but that's more or less irrelevant with feature toggling, staged roll-out and continuous production monitoring.
It's totally possible to completely avoid mocking http endpoints thus making these tools completely obsolete.
await fillIn('email', 'polly@netflix.com');
await fillIn('password', '@pollyjs');
But sometimes that database is down, or really slow.
Polly says "browser, don't talk to the database anymore, instead here's what the database said last time".
So, yes, both Selenium and Polly poke DOM elements, but Selenium stops there, where as Polly does that + as well as tricks the browser into going through the whole test without making a real call to the database (assuming it has a previous recording of "what the database said" for that test).