I really had no desire to get invested into this, but since this gets in my feed so frequently, here we go. I've read relevant posts on Daniel Berntsson's blog, i.e. the individual which this should really concern; instead of Mullvad as a whole. I understand the donated money possibly came from Mullvad subscribers, and if that leads you to unsubscribe, by all means do so; but stop bugging the rest of us.

It seems he donated to ÖP because they criticize excessive taxation, that is believed to typically end up enriching an (upper-)middle class (the "transferiat"), that gain additional support for (self-serving) policies, from lower (working) classes that are dependent on (welfare) subsidies. And given that immigrants are typically of the latter class, it's believed to further reinforce the transferiat's position: by a growing dependent class voting in favor of pro-transferiat policy.

This isn't to say he agrees with (stricter) immigration policy, nor with the other positions taken by ÖP, or any other political party for that matter. Instead, he adheres to a "libertarian anarchist" ideal, where "people can ... organically and voluntarily federate together at any scale". Rather than states appropriating capital from citizens, to only partially redistribute effectively in the form of common goods, which involves protection thereof through immigration policies for example; often to the detriment of people, including him personally.