It's a rabbithole, and you might not be personally satisfied with the standard of evidence. Regardless, every other VPN has some evidence of partnering with some combination of state actors, data brokers, ad networks, or unexplained unnecessary compromising requirements etc. Mullvad has no such substantiated allegations against it.

There are many VPN services that begin by reselling white-label VPN solutions, such as provided by NordVPN,[1] because it's cheaper and easier than building your own globally distributed high-capacity, low-latency network. Many suspect Proton VPN did so[2].

[1]https://nordvpn.com/white-label/ (old link, can't find an archived version of the page) [2]https://archive.is/iZ2l2