Sure. It's not hard to verify, in the user ui, function calls are very transparent.

And in the api, all of the common features like maths and search are just not there. You can implement them yourself.

You can compare with self hosted models like llama and the performance is quite similar.

You can also jailbreak and get shell into the container to get some further proof

this is all just guesswork. it's a black box. you have no idea what post-processing they're doing on their end