Response: Attempts to portray atheism as a type of religion typically depend on misrepresenting atheism, religion, or both. This particular myth is focused primarily on misrepresenting the definition of religion (though atheism isn't represented very accurately, either).
What's amusing, though, is that is misrepresents religion in a way that many believers, in other contexts, would object to. And for good reasons, too!
Atheism, Proof, and Knowledge
Taking the smaller misrepresentation first, is atheism a belief that cannot be proven and cannot be known? No. Defined broadly as the absence of belief in gods (weak atheism), atheism isn't even a belief at all, much less a belief that cannot be proven or cannot be known. It might (in theory) not be rational and the arguments for it might (in theory) not be sound, but it's not a belief lacking proof.
Defined narrowly as the denial of the existence of gods or the belief that no gods exist (strong atheism), atheism would at least qualify as a belief. Whether it's a belief that cannot be proven and cannot be known is a tougher question to answer because it all depends on how one defines "god." Under certain definitions, disproof of "god" would be quite possible; under other definitions, it wouldn't.
What's key is that it cannot be assumed that just because a person believes that some god doesn't exist that they cannot possibly prove that it doesn't exist and cannot know that it doesn't exist.
People who assert such things are always assuming some particular definition of "god" and thus prejudicing the debate unjustifiably.
Religion, Proof, and Knowledge
Turning to the more significant misrepresentation, is "religion" defined as "belief without proof or knowledge"? Sometimes faith is defined in a manner like this, but religion never is — at least, not by any reputable, scholarly, or academic source. People who define such a broad, varied, and multifaceted cultural phenomenon like religion in such a superficial and simple-minded manner cannot and should not be taken very seriously.
This is not to say, of course, that there isn't any debate about what religion is. On the contrary, there is quite a bit of debate on that matter. This, however, is a demonstration of just how complex religion is. Scholars recognize that there are a lot of different elements that go into religion and it's common for this or that element to be emphasized as an essential and defining one (like worship, for example). Belief without proof or knowledge, however, is never picked as an essential and defining element.
Religion and Belief
It's not just scholars who fail to see religion in such a simple-minded manner. Believers also often recognize that religion is much more than this. I doubt that a single Christian, Muslim, Jew, Hindu, or Buddhist would agree that what makes their religion a "religion" is nothing more than that they believe something without proof or knowledge.
That's assuming, of course, that they would even agree that they do believe something without proof or knowledge. Some probably would, but many others would insist that they do have proofs and/or that they do have knowledge. Outsiders would likely disagree, but do we really want to get into outsiders deciding what defines another person's religion?
Thus even if atheism could be legitimately described as believing something without the possibility of proof or knowledge, this couldn't justify characterizing atheism as a religion. If believers would refuse to accept such a definition as applying to their religion, why should anyone accept it as definition of religion to use with atheism?