Well he is dealing about 92 damage assuming everything hits and all saves are failed.
If we use Mike Shea's 7.5 Dmg per CR (which I think is a good guideline) Zargon's damage is on par with a CR 12-13 monster.
For CR 17 we should be looking at closer to 127 damage. Zargon is about 70% as effective as he arguably should be.
I reckon any typical Level 20 fighter with average gear beats that Zargon one-vs-one.
Assumes he hits with the horn (which is a very fun attack - I'll give them that).
Overall I think its one of the better designed WotC monsters, but again, the damage just seems low and in many cases trivial. His 'big' recharge attack deals 38 damage, 19 on a save. The breath weapon of an Adult Red Dragon (which has the same CR) deals 63/31.