The origin of using so like this is Valleyspeak.
Valleyspeak
Generally, it replaces very but with a noun.
One might say: Wearing those shoes is so summer.
Instead of: Wearing those shoes is a very summery thing.
The spread of so and like in American speech is omnipresent, especially with young people.
You can use so with any noun.
- He is so Los Angeles.
- She is so New York.
- They are so fashion.
Older generations would probably say: They are so fashionable. [adjective]
The trick is the noun instead of an adjective.