If I understand the OP well, the question is, how can Mozilla justify charging twice what Mullvad charges, and lock you into a 12-month contract, for what is largely a repackaging of Mullvad.
So just use Mullvad instead. For those who want to support Mozilla's efforts, this is a good way of doing it. Think of your extra $5 as a sort of donation to them for building such a wonderful browser and browser ecosystem (Lockwise, extensions, Pocket etc)
But firefox is developed by mozilla corporation, and donations to mozilla foundation doesn't fund firefox development. It goes to various advocacy/activism causes.
While executives earn money like at any company, most of their budget is spent on Firefox. 2% of the Mozilla Corporation revenue goes back to the Mozilla Foundation.
That downplays the remuneration of the principle Moz exec considerably AIUI. Mitchell Baker's remuneration from Moz was >$2M last I heard (a few years back [see eg https://calpaterson.com/mozilla.html]), I can only assume it's more now.
There was also a report Baker had a >$1M "stipend" (uncharitably a bung) from Google, though I've been unable to refute/corroborate it.
For clarification, I believe he is saying you pay the same price as a mullvad subscription if you lock into a 12 month subscription, not that you pay double the price and are locked into that.