Larry directed and was part of the writing process, often being the executor of ideas and notions that passed or failed, but to say written primarily by him sort of devalues the stories/experiences and input from the staff which were often the basis of entire episodes, not just added snippets on an already given idea or episode written directly by either Larry or Jerry.
There's a give and take. Like for instance, The Big Salad premise was indirectly inspired by David's constant grumbling about who should take ownership of a compliment given towards a parcel of food, and while Larry didn't write or include that, he was the inspiration for it solely and it was written for George. Festivus, which has nothing to do with Larry, but the premise was absurd enough that it would fit with George's character, of which Larry approved.