Moderators wear many hats but their main role is to enforce community standards. That's not quite the same role as dispute mediator, because not every dispute between users is going to violate community standards.
In the case that you describe, any intervention by the non-expert moderator should explicitly address the behaviors that violate community standards and explicitly reference those standards. It's unnecessary to single out particular users provided the intervention is visible to all participants in the dispute; the focus should stay on behaviors rather than individuals.
There's no need to be an expert in the topic because whether one side or the other is correct is not relevant in a moderation context. Decorum and scope are relevant, and what the moderator should focus on.
Even if the moderator is in fact an expert in the topic at hand, the act of moderation should be reasonably separable from the communication of an opinion (expert or otherwise) on the disputed topic.
Here's an intervention that would be less effective:
You're wrong CoolGuy, the mecha-coelacanth presupposes an occipital ridge. Now drop it.
- CoolGuy might get the message that he's been ejected from the conversation based on the moderator's expert opinion of the topic of discussion, because the intervention doesn't explicitly identify the reason for the warning.
- This intervention doesn't explicitly identify the community standards that were violated. Even if the moderator is responding directly to CoolGuy's content (as by quoting his words, or comment threading that makes the parent clear) he may not know exactly what he did wrong.
- This intervention singles CoolGuy out, which could result in other users feeling like they have free license to be hostile to CoolGuy, when what you want is for nobody to be hostile.
If the moderator is intervening because CoolGuy has been antagonistic and insulting toward another user, here's an approach that would be more effective:
The tone of this discussion is getting inappropriate. Please keep in mind that this is a public forum, and the rules of engagement ask that members behave respectfully with each other at all times, even when we disagree. That means no personal attacks and keeping the sarcasm to a minimum.
As far as the ridge is concerned, I'm pretty sure it's always occipital when you're talking about a mecha-coelacanth, and here's why...
- This approach refers to existing community standards as the basis for moderator intervention. Ideally, the link leads directly to the relevant bit about respectful behavior.
- This intervention identifies exactly which behavior was inappropriate and suggests ways to behave appropriately.
- Even if CoolGuy is the only user instigating trouble, this intervention doesn't single him out, because the standards apply to everyone.
In this approach the moderator's official statement is separated from the moderator's personal or expert opinion by a new paragraph and a signal phrase ("As far as the ridge is concerned...") to mark the transition. This is what makes the act of moderation reasonably separable from the communication of the opinion. If the moderator doesn't have any technical expertise, or just doesn't want to participate in the debate, it's perfectly fine not to offer an opinion on the topic at hand.