Upvote:3
As the answer you mentioned suggests, the majority of Calvinists and that which is taught by the reputable Wayne Grudem in his Systematic Theology and many others, is that, in 2 Peter 3:9, "all" means "all", and not just "Jesus believers." The verse is definitely referring to salvation, so it is not talking about the heart and mindset of continual repentance that we should all have about all sin.
It would not make sense for this verse if all was referring to Jesus believers. That seems superfluous, and like you said they are not in danger of perishing, so to speak.
However, Calvinists could say that verse is referring to the elect, prior to salvation. Because the elect, prior to salvation, would still need to repent and be saved from perishing, but they are the elect from eternity past, so the title of "elect" would be accurate. This is how that interpretation is at least coherent but doesn't adequately explain the use of the term "all" or "everyone" without hermeneutical gymnastics.