This is not only not possible to build in any reliable and maintainable way, but "[w]e are the most accurate AI text detector that exists" is an outlandish claim.
None of them are very good, so I don't think this claim is very outlandish.
Also, are you sure it's not reliable or maintainable? Obviously you can't publish one model and expect it to work forever but we have pipelines to continuously augment our training set and we can add new LLMs as they come out.