I think market value would dictate that he is probably worth closer to 12M imo, but that's because market value is always shifting. Nylander is the closest, and most recent, comparable and is likely the one Pettersson's agent will use.
Market value for franchise players shifts, and what some guys get is also dependent on when their contracts expire. Matthews just raised the bar for franchise, top of the league players with his 13.25M contract. McDavid will shift it again his deal is up. Draisaitl will shift it before McDavid, his deal is up after next season.
Some folks will point to Pastrnak, that's been an ongoing discussion within this thread as well. But Pasta signed last season on March 2nd, 2023. He didn't have the incoming cap jump to bump up his pay, Pettersson does. The rising cap will always disrupt past contexts, because contracts for top players will always rise alongside the cap. It's the same thing with MacKinnon, who folks also point to. But the context there is he signed under a Covid flat cap that wasn't seeing sizeable jumps back on September 20th, 2022. He didn't take a discount either. Folks will argue Pettersson shouldn't get more than either player as if things are black and white, when context will tell you than things aren't black and white.
Folks argue that 12M is for top of the league, top 5 tier players. Pettersson is already in the top of the league bracket, but 12M may not be top of the league money 2-3 years into Pettersson's next deal.
Pettersson will almost certainly get the same or more than Nylander whether folks like it or not unless his deal is like 2-3 years, and even then he might.