You can test this locally yourself with mitmproxy, opensnitch, or whatever.
You can try building the (supposedly) open-source apps you use from source.
Everyone opining here should MitM themselves every now and then. If not for your own security then maybe to make sure you're not participating in psyop when opining online and resharing hearsay or old truisms.
The difference is Chromium feels like Chrome if that's what you want to use and trust, it does not feel like Vivaldi and that's basically all that's provided here.
Confusingly, that page only provides the changes to the Google Chromium source that allows their UI to run. (I'm not sure it would be easy to discern this without already knowing the source is not fully open.)
I don't trust them one bit. There was that telemetry analysis that showed Vivaldi as a very noisy browser.