No, you're wrong in the sense that they literally collect 0 data on your Mullvad BROWSER usage. It can't be another data point because Mullvad likely doesn't even know if you're using Mullvad Browser, let alone what you're doing with it.
Your point is only valid in the case of internet services, which a browser isn't. A browser is a piece of software that can be replaced in a minute if let's say Mullvad's whole development team and CEO went full batshit crazy and placed backdoors in the public (and FOSS, so it can be forked) codebase, unlike e-mail etc. And your Google example is invalid because you can't compare something like Google Chrome to Mullvad Browser.
You could try using Cromite just for this.