I agree with you; I am abusing terminology.
For the record, though, I do think we have a bias against theism in the sense that you've described. I understand why LessWrong might choose to consider the "does God exist?" question settled, but we go further than that. We frequently discuss how terrible religion is, and applaud efforts to promote atheism, despite the benefits to happiness etc. that religion provides.
I think it's understandable that we have such a bias. On LessWrong, we value truth and truth-seeking; this goes far enough that it's almost a moral value. When other groups actively discourage truth-seeking, we oppose them.
I don't know anything about social psychology, but my experience with other social sciences suggests that liberal, humanist values are deeply ingrained in their system. What I mean is, the social sciences are not just truth-seeking engines looking for facts about humanity. They have their own moral values attached (e.g. reducing discrimination).
Just as we on LessWrong are reluctant to engage in discussions with people who oppose truth-seeking, social scientists may be reluctant to engage in discussions with conservatives, because conservatives tend to hold moral values that are actively opposed to the social sciences' agenda.
This article suggests implicitly that the social sciences should be about truth-seeking, not about promoting some political/moral agenda. The willingness of social scientists to admit their bias against conservatives suggest that they feel otherwise.
We frequently discuss how terrible religion is, and applaud efforts to promote atheism, despite the benefits to happiness etc. that religion provides.
Those benefits are primarily based on the communities. If we build similar secular communities, like the link I gave suggests, then there wouldn't be a special benefit to religion.
Summary: Current social psychology research is probably on average compromised by political bias leftward. Conservative researchers are likely discriminated against in at least this field. More importantly papers and research that does not fit a liberal perspective faces greater barriers and burdens.
An article in the online publication inside higher ed on a survey on anti-conservative bias among social psychologists.
The link above is worth following. The problems that arise remind me of the situation with academic and our own ethics in light of this paper.
I can't help but think that self-assessments are probably too generous. For predictive power of how an individual behaves when the behaviour in question is undesirable, I'm more likely to take their estimate of how "colleagues" behave than their estimate of how they personally do.
This shouldn't be surprising to hear since to quote CharlieSheen: "we even have LW posters who have in academia personally experienced discrimination and harassment because of their right wing politics."
While I can see Lammers' point that this as disturbing from a fairness perspective to people grinding their way through academia and should serve as warning for right wing LessWrong readers working through the system, I find the issue of how this our heavy reliance on academia for our map of reality might lead to us inheriting such distortions of the map of reality much more concerning. Overall in light of this if a widely accepted conclusion from social psychology favours a "right wing" perspective it is more likely to be correct than if no such biases against such perspectives existed. Conclusions that favour "left wing" perspective are also somewhat less likely to be true than if no such biases existed. We should update accordingly.
I also think there are reasons to think we may have similar problems on this site.