I don't think that's the case at all.I still resent this implication. It means that the atheist/s considers nearly all of humanity mentally ill while he himself, themselves are not. It speaks for itself who is thus and therefore mentally ill. Frankly, that subject should be stricken from this website as inflammatory and insulting to so many. It should be reported and enforced by the moderators.
Distinguishing between genuine, let's call it, enthusiasm for one's religious beliefs and mental illness is a worthwhile endevour. One has to seperate and address those who are fervent believers and those who just so happen to have a mental illness. Because that person may be hearing voices telling them to do awful things. And may be accidentally encouraged to follow through on such actions by well meaning congregations unaware of said person's condition. Giving credence to the voices by explaining it, unintentionally, as God speaking to that person. I don't think that religious ferver equates to mental illness. Nor did I get that from the OP. But that does not mean that there can't be those suffering from mental illness becoming wrapped up in religious ferver, for whatever reason. (Upbringing, culture, family ect.)
Even discussing mental illness is a worthwhile endevour. Because you begin to peel back the stigma and dismiss such things as shame or embarrassment for those suffering. Because mental illness is not necessarily someone being hauled off in a straight jacket to the "nuthouse." Plenty of people live, for lack of a better term, normally with mental illnesses. They just happen to need a little bit of help from time to time. Whether that be coping strategies recommended by professionals, being in therapy or even pills.