The concept of ‘moat’ comes out of marketing - it was a concept in marketing for decades before Warren Buffett coined the term economic moat. Brand moat had been part of marketing for years and is a fully recognized and researched concept. It’s even been researched with fMRIs.
You may not see it, but OpenAI’s brand has value. To a large portion of the less technical world, ChatGPT is AI.
Nokia's global market share was ~50% in smartphones back in 2007. Remember that?
Comparing "brand moat" in real-world restaurant vs online services where there's no actual barrier to changing service is silly. Doubly silly when they're free users, so they're not customers. (And then there are also end-users when OpenAI is bundled or embedded, e.g. dating/chatbot services).
McDonald's has lock-in and inertia through its franchisees occupying key real-estate locations, media and film tie-ins, promotions etc. Those are physical moats, way beyond a conceptual "brand moat" (without being able to see how Hamilton Wright Helmer's book characterizes those).
You may not see it, but OpenAI’s brand has value. To a large portion of the less technical world, ChatGPT is AI.