I think the problem is that chrome and firefox are developed for general usage, and as a result, have to deal with general usage constraints. For example, spectre and process isolation mentioned in another blog post here in the comments. I had a project in the past where I needed a browser with good JS support, but that does not make any http requests to any resources not in address bar or in the page rendered at that address. I did not find single browser based on firefox/chrome that fulfilled that requirement. Even the most privacy-focused projects still made continious requests to some mozilla resources (if i remember correctly, it was something about tls or maybe something else). So seeing someone creating an engine from scratch gives me hope that such browser might exist one day if I ever will need one again.
There's a difference between privacy and stealth. Modern browsers need to do things like captive portal sensing, ocsp checks, etc. If you want to disable all requests, you'll need to spend time cutting legitimate features. Or run it in an external sandbox which removes all networking access?
