It's not. According to Dictionary.com the definition of religion is this:
a set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs.
Alain de Botton seems to regret that atheism is not that, and while he wouldn't accept the superhuman agency he seems to want to co-opt some of the "devotional and ritual observances".
On Justin Brierley's Unbelievable? radio programme today Alain de Botton stated that he doesn't really care for the nitty gritty details of science, and it seems that this barely concealed disdain for the hard facts of reality could be at the root of his less-than-rigorous approach to truth — an approach that sets him apart from other philosophers such as Daniel Dennett and A. C. Grayling. And of course Alain de Botton is far too nice to come out with full-blown condemnation of religious belief like Richard Dawkins and his ilk are wont to do. (His niceness was on full display in his conversation with James Orr today, but he was, almost literally, on his own ground.)
Alain de Botton already appears to be borrowing aspects of religion, such as the insistence that an absence of religion will inevitably leave a void requiring to be filled. This is not so, in the same way that removing a cancerous tumour from the body does not require something in its place.
His idea that there ought to be a community for atheists seems to me — someone who has not read his book — to be fundamentally misguided. There is already a community for atheists and people of a secular humanist turn of worldview; it's called humanity. We secular humanists (I count myself among them) can do what others do when when they don't go to church, such as attend or partake in sports, go down the pub, go to the movies, theatre, sightseeing, evening classes, quiz nights, museums, art galleries — or even skeptics conferences if we are so inclined.
All of these are communities of different kinds; pick one (or more) as you like. There's no need for something to serve as an ersatz church.
Alain de Botton gave a TED talk recently on the theme of his book: