Well how is it his greatest achievement when he was involved in approximately 30% of it ? The majority of the show was someone else's achievement. Granted, he co-created the show and directed a few choice episodes, but he was involved surprisingly little. I don't consider it 'David Lynch's Twin Peaks' just as I don't consider it to be his towering achievement as most of it came from other people's creativity. Mulholland Dr, Lost Highway, Eraserhead, Inland Empire, those are projects he had full artistic control over.
He gave birth to the show (and helped destroy it) but barring a few guiding hands along the way and co-creating the tone other people raised it.
Barring when he acted in it, Lynch had direct involvement in a third of the episodes and didn't even attend production meetings for half of them. Mark Frost was essentially the show runner and that final episode was already written when Lynch decided to direct it.
Is it nitpicking day? I must have missed that.
The stuff that he created for Twin Peaks is, in my opinion, his best work. Is that okay for you? The below list is what I liked about his contribution, it's not limited to his involvement.
The Pilot (possible "direct to video" feauture film with alternative ending): Writer, Director, Executive Producer
Episode 1.2: Writer, Director, Executive Producer
Episode 1.3: Writer, Director, Executive Producer
Episode 1.4 - 1.8: Executive Producer
Episode 2.20: Writer, Director, Executive Producer