It's open source in the way that Android is open-source. It's a corporation's product that they allow people to contribute to and use freely because it's advantageous to them. They get free labour from other companies and individuals, and they get to sneak their ad network and spyware into every single alternative browser that's based on chromium.

Open-source software is not inherently good or based just because it's open-source, just like a Chinese company isn't inherently good or based just because it's based in China.

Microsoft has similarly used the Embrace, Extend, Extinguish model to kill tons of alternative software, and Google has done the exact same thing with Chromium. It's killed off every alternative browser aside from its cousin Safari and Firefox. Everything else but for the small handful of Firefox forks like librewolf are just Chromium with some UI changes and possibly built in extensions.