I'd think the Devils internal "cap" is on cash, not AAV. He's owed $8 million next year, with $6 million of that as a signing bonus.
For 2021, the last Stanley Cup final game could be played on July 9. Signing bonuses are due July 1. They were actually paid out on July 1, 2020 despite the 2019-2020 season not yet being complete.
If the signing bonus payout remains July 1, Subban will only be due $2 million in additional compensation for the 2021-2022 season. I think Subban will be worth $2 million to the Devils.