They use OpenAI's API with the possibility to set a custom URL, so... done. This makes iTerm compatible with any LLM as long as they implement the same API.
OpenAI's API is the de facto standard for now, it's not up to iTerm to define a standard.
It doesn't look like there's the ability to change the URL. The only options are to change the API key, model name (from a predefined list), the prompt, and the token limit.
I hate "AI integration" rubbish as much as everybody else. But in this case the author did it well - it's disabled by default, it won't work unless you decide to paste your key and so on.
OpenAI's API is the de facto standard for now, it's not up to iTerm to define a standard.