In a strictly logical sense you are right. An atheist saying there definitely is no god is making a statement that cannot be proven. The same is not true of a theist saying there is definitely a god. That is a statement that could be shown to be true - all it needs is evidence of the existence of a god. The fact that there is no such evidence doesn't make the statement false, it makes it unproven.

Of course, any monotheistic believer has to accept the possibility that the gods that they do not believe in might exist otherwise they are applying different logic to themselves and believers in other gods, which is at best inconsistent and is exactly the same arrogance of which they accuse atheists.

For myself, I am a pragmatic atheist - I have no reason to believe there is a god and the existence or otherwise of a god makes no difference to m life, so I might as well act as if no gods exist.