The repeated code on the steps is actually the more interesting tell to me. An artist would vary that deliberately for visual interest. A model just tiles what it learned looks like "code."
That said, the pencil sketch theory is compelling. Hybrid workflows where a human does the composition and an AI handles color/rendering are increasingly common, and they produce exactly this kind of uncanny result — strong underlying structure with strange surface artifacts.
Whether it is or isn't AI, the irony of a documentary about a language whose community deeply values craft and intentionality potentially using generated art for the thumbnail is at least worth a raised eyebrow. Not outrage-worthy, just... a little incongruous.