When they are focusing on just being an API provider then they will be in a market with (long term) razor thin margins and high competition - most likely unable to build a deep moat. But if you can shape customers habits to always input "chatgpt.com" into the browser whenever they want to use AI then that's a very powerful moat. Those customers will also most likely be on a subscription basis, meaning much more flexibility in pricing and more rent for openAI (people using it less then what OpenAI calculates for subscription costs).