Retention is dictated by Swedish laws, not refund policies.
Like many in this thread have suggested, it is unclear how this helps Mullvad rid of data they’re legally mandated to store & share: Looking for VPN services that are not based in the West - #9 by ignoramous
Public VPNs are highly lucrative. See this discussion: Cheaper VPN service recommendations? - #30 by Lukas
The account ID can be tied to payment and hence user identity depending on the payment method? It isn’t a blanket “no-trace”.
It is even worse for Mullvad to sell a business decision as some privacy related move. Though, I wouldn’t be surprised if they did.