In the Elephant, Peter Johnston remarks that internal categories may be regarded as simplicial objects that “preserve all limits that happen to exist in $\Delta^{op}$“ (I guess you might call this a flat functor). This is because the join in $\Delta$ is a limit.


Does a similar statement exist for the symmetric simplicial set/groupoid correspondence? It’s clear that the nerve of a groupoid yields a symmetric simplicial set, the question is then whether or not they correspond to flat functors on the site for symmetric simplicial sets.



Groupoids as models of symmetric simplicial sets
You can definitely characterize groupoids as presheaves on $Fin\_+$ preserving some colimtis (i.e. sending some colimits in $Fin\_+$ to limits in Set). In fact Groupoids are the presheaf on $Fin\_+$ that preserve the colimits comming from $\Delta$.


However, the Category $Fin\_+$ has much more colimits than $\Delta$, in fact it all non-empty finite colimits. Groupoids do not preserves most of these colimits.


For example, in $Fin\_+$ you have $\{1\} \coprod \{1\} \simeq \{1,2\}$ , but given a a groupoid $G$, $G(\{1\})$ is the set of objects, and $G(\{1,2\})$ is the set of arrow, so the isomorphisms $G(\{1,2\}) \simeq G(\{1\}) \times G(\{1\})$ means that your groupoid has a unique arrow between any two objects.


In fact, I claim that the colimits in $Fin\_+$ that exists are exactly the finite non-empty colimits, and the category of presheaf preserving them is equivalent to the category of sets. In terms of the usual description of groupoids as presheaf on $Fin\_+$, these corresponds to anti-discrete groupoids.


