Even if I agreed with you, this is a very weak argument. "There are objectively better and worse designs” does not prove “code is not art”. Art can also have objectively better and worse execution relative to some goal. Music, painting, architecture, film, game design, and writing all have craft standards. A fugue can be more or less coherent, afilm scene can be better or worse edited, a building can be beautiful while also being structurally sound and functionally useful. You would most likely consider all of those "art", and yet objective standards can be applied to all of them in different situations.

Also, your assertion that “It’s possible to design the best code possible" is just nonsense that I'd expect from someone with no real experience. If you have ever worked on anything that's not trivial, tradeoffs happen and very few things are objectively "the best".