> it can't be true that it exists for us

You're saying it as if you assume some external entity judging whether something exists for somebody or not.

As an atheist you would acknowledge that the entire concept of "existing for something/somebody" is entirely a construct of human mind, which human mentally applies to the observable universe around them. So for an atheistic human mind, everything exists for human, as there's nobody else to exist for.

We're putting too much pressure on the word "for", here. There are no great truths to be found at this level of linguistic imprecision .