Well the closed source bit is that you also cannot reproduce the binary yourself (so we don't actually know if there is other code in Chrome), and Chrome presum

Well the closed source bit is that you also cannot reproduce the binary yourself (so we don't actually know if there is other code in Chrome), and Chrome presumably adds a load of (non open-source) code around sending your browsing data and history to Google servers. The open source version also doesn't include any way to do automatic updates, sync with your google account, or have access to the extensions store. None of this is inherent to the open source version, but presumably it benefits Google to push their closed-source version which includes all their secret-tracking-sauce and allows them to avoid people forking the project entirely to remove ads. Firefox manages to support all different video types while having reproducible builds (see: OpenH264) :)