Let us start with the definition; a religion is "a group of beliefs". Humanism has beliefs, as many things do, and as such qualifies. It doesn't need any sort of pseudo-authority to say it is, as the only thing truly necessary is the idea that one can believe certain things, and if one can classify those into a group then something is a religion. As a humanist myself, I do find some claims ridiculous at best, to be blunt. Anyone willing to challenge me will be accepted, of course.
Religion is defined as "the belief in and worship of a superhuman controlling power, especially a personal God or gods." Humanism has no superhuman controlling power. It has now supernatural beliefs. If you're going to go off any group with a belief system is religious, then so are Capitalists, Communists, people who like James Bond and Vegetarians.
Humanism doesn't have a system (e.G. Any kind of rules like other religions), doesn't have a deity or anything of worship, doesn't have beliefs in the supernatural, or even a creation story. If you look at the definitions on Merriam Webster, Dictionary.Com, or the Oxford Dictionary, many of these are requirements for a religion, the closest Humanism gets is "a cause, principle, or system of beliefs held to with ardor and faith" in MW, although I wouldn't say Humanism is based on faith.