Douglas Kennard’s article Petrine Redemption: its Meaning and Extent gives an overview of Peter’s concept of redemption and then dives into 2 Peter 2:1’s statement ‘denying the Lord that bought them’.
For Peter, Christ’s death is substitutionary in nature and is like a sacrificial lamb. Kennard argues that for Peter, redemption is not a payment to someone (either God or the devil). Rather it’s simply accomplished. Redemption is a onetime action not a continuing enablement. Redemption is out of a corrupt, sinful life and requires the redeemed to live differently. Redemption for Peter is not equivalent to salvation for Paul. One can be redeemed but not ultimately saved.
Regarding 2 Peter 2:1, Kennard defends the view of the apostatizing of unsaved knowers of the truth. Kennard understands Christ to be the ‘Lord’ (despotes) and the redemption (agorazo) to be soteriological. That 'Lord' refers to Christ can be seen in that Christ is the redeemer (1Peter 1:18-19) and since despotes is translated adonay in the LXX, which Peter understood to be Christ (Acts 2:34-36) and finally since in the parallel account in Jude 4, despotes refers to Christ.
Peter reveals three groups of redeemed individuals: 1) believers growing in salvation (2 Peter 1:4-6), 2) those who began to change but stopped (2 Peter 1:9-11) and 3) false teachers who barely escape lusts and then are enticed back (2 Peter 2:18-22). These false teaches are the ones redeemed in 2 Peter 2:1 and they changed their lives for a while but fell back into their sinful ways. These false teachers were recipients of redemption, but not other soteriological works needed for salvation. So while redemption extends beyond the elect it is a limited redemption that is actually applied to all who have their lives transformed.