I mean they did say few. Generally speaking, every browser is basically safari (WebKit) on iOS and apple doesn’t allow support for 3rd party browser extensions (least natively, Orion supports this somehow). So you’re already limited in that regard. If you don’t use safari , a browser like FF + VPN is IMO a better experience. You also have the option of just using wireguard and controlling your traffic at home/VPS if you’re into that.
WebKit might be open source but the browser deployed by apple is not. That’s like saying chrome is open source. They both use open source engines.