Actually, the Forge of Foes baseline for CR17 is the following:
CR 17
AC/DC: 20
HP: 246
Atk/Prof: +12
Damage Per Round: 107
It's preeeetty close, even closer if Zargon hits 3-4 characters with its breath weapon. I do think the counterspelling and anti-magic horn attack will make him a lot more tough. I see a lot of folks saying fighters would obviously have a way to deal fire damage, and sure, in a campaign, a party might have learnt about his weaknesses. Otherwise, they will need to deal that fire damage every round, which can be more difficult than you think.