This stat-phys thing (cluster expansion) and that qft thing (cluster decomposition) are related by both referring to the notion of cluster, being "a small group or bunch of something.";
the former referring rather to cluster in the sense of a "group" of some particular (small, or large) number of members,
with particular and primary attention given to pairwise membership (thus not including clusters in the degenerate sense of having only one member),the latter referring rather to cluster in the sense of a "bunch" of members that are coincident,
thereby characterizing pairs of members of distinct bunches as not coincident (and including the notion of a bunch in the degenerate sense of having only one member).
(More might be said, 'cause that's really my kinda bag ...)