Huge brand moat. Consumers around the world equate AI with ChatGPT. That kind of recognition is an extremely difficult thing to pull off, and also hard to unseat as long as they play their cards right.
Brand loyalty and users not having sufficient incentive by default to switch to a competitor is something else. OpenAI has lost a lot of money to ensure no such incentive forms.
Moats, as noted in Google's "We Have no Moat, and Neither Does OpenAI" memo that made the discussion of moats relevant in AI circles, has a specific economic definition.
https://www.goodreads.com/book/show/32816087-7-powers
It has branding as one of the seven and uses coca cola as an example.
You may not see it, but OpenAI’s brand has value. To a large portion of the less technical world, ChatGPT is AI.
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).