And if I do trust mullvad, why wouldn't I buy access from them directly? The only reason to get the Mozilla VPN is, to help them to a little commission for ideological reasons.
If I were to use CloudFlare's VPN service of Comcast's DNS, I'd use it through Mozilla because their contracts stipulate extra protections.
"Mozilla has a legal contract with Google that prevents them from using our Google Analytics data for mining or from sharing it with third parties, among other privacy-protecting provisions."
Because far more people are familiar with Mozilla than Mullvad. Knowing they're the same service is just insight that can save a few euro.
You're right that they have indeed more brand recognition. Basically, the value proposition is: Out of all the VPN providers, we already did the research for you, to pick one that's trustworthy.
They have only spent effort on a reseller partnership till now, so if it fails to pickup not much Engg effort has been wasted either