What is germane here is that the article says photon-photon interactions in the absence of matter will thermalize radiation. In my view, that is an unsupported statement. The Tolman reference is to a certain derivation of the H-theorem, which might be germane, but does not specifically address this mechanism. Waleswatcher believes it does apply, and takes the view that his opinion is quite sufficient without any need for further support. Of course, his opinion may be correct, but WP policy allows such opinion to be challenged. I do not choose to do this, but others might.