I actually disagree with this. If we look at some of the features Mullvad has implemented in recent years from DAITA to post-quantum servers, while not forgetting their anonymous account creation, it seems clear to me that when it comes to privacy features, Mullvad is currently ahead of Proton. There are also smaller things, such as better kill switch on macOS, which all together make the difference quite apparent.
When was the last time Proton introduced some new innovative privacy feature for their VPN? For me it seems that their focus has been on other things, such as growing their server network and enabling streaming support for more services.
Having said this, I have recently switched from Mullvad to Proton because I got a great deal on Black Friday, and see Proton as one of the top VPN providers, but I still don’t think it is the best one privacy-wise.
Mullvad has actually more servers and locations than IVPN who has the smallest network from these three.