My original point to OP was that none of the benefits listed are inherent or unique to theism, and thus constitute a poor basis on their own for the stance being advocated by OP. I think that generally stands, and we are engaged in a different and more nuanced debate to which my replies are as follow:

Do you suggest that their churches were a negative factor here, or is it more likely that the individuals in question had it lesser across the board?

The theists I have known who have lacked or experienced the benefits in question to a lesser extent than myself generally did not do so on account of any single variable. There are a lot of reasons why organized theism may not generate the purported benefits at all, let alone to an equitable degree as for atheists: exclusionary faith practices (sexism, racism, homophobia, etc.), encouraged reliance upon externalized coping mechanisms rather than emphasis on independent personal development, etc.

Do you suggest that simply being a theist intrinsically limits opportunities for other forms of support?

On a certain level, I do think that theism imposes inherent restrictions upon persons that they otherwise would not experience. Most notably, I think it retards personal growth and development and fosters an over-reliance upon the deity figure and other ideas necessary to faith. Emotional and cognitive maturity can affect the strength, stability, and prevalence of our connections with others. I think theism is also more disposed to insulate prejudice, and that this can limit opportunities for connecting with other persons. Etc.

My point is that regardless of the overall quality of each support option available, theists have at least one additional potential support option beyond that of what atheists have.

I understand, and my point is that that rationale applies in both directions rendering it a draw (i.e. atheists do not have access to theist bonding, but theists do not have access to atheist bonding). Your sole premise for dismissing that argument seems to be that you have not experienced atheism as a comparable support option; that would be a like a theist dismissing theism because they did not personally experience it as a support option. It is quite simply an inadequate basis from which to draw a conclusion, in my opinion.

It seems to me that bonding over a lack of belief in a god is fundamentally the same as bonding over a lack of belief in santa clause.

Hardly. For one, theistic beliefs are commonly held as real whereas Santa Claus is not. The shared experience of being a minority, with varying degrees of accompanying marginalization, can absolutely be the basis for forming connections, and it is fairly common among minority populations. Generally speaking, shared identity alone regardless of context can be a strong connecting factor for some people.

Were you raised a theist, [...] but maybe I'm off base.

I was raised a theist, but never was one. So, actually, I generally have rather little in common with respect to a lot of other atheists who experienced it in one of the two ways you mentioned. For me, my shared bond has been more in the comfort of knowing we share (or lack) similar assumptions or values about the way the world and life are to us.

We'll just have to disagree. [...] This seems to be speculation that is predicated on not wanting to admit that there are any current benefits to theism at all. Are you certain you are looking at this objectively?

Yes, and I would contend I am doing so far more than you are. I have expressly acknowledged and not once denied that there are benefits to theism, repeatedly. Your implication to the contrary is misrepresentative, and you have also falsely declared my views to be speculative even though I provided a supporting rationale that you did not directly refute.

That rationale once more: The history of evolution is replete with beneficial byproducts for which attributes were not initially selected. My express point is that your argument is assuming any and all benefits of theism to have been primary selective variables, rather than being beneficial byproducts. Your assumption is particularly questionable with respect to benefits which are non-unique, because if those benefits were not unique to theism then there would have been absolutely no reason for evolution to have favored theism due to those benefits.