I guess it really just doesn't do anything. Especially since nothing can join or leave there unit. They probably got it from a lore stand point since they are still orc's even if they can't have more models in there group.
A lot of rules in 40k are included simply for 'completeness' sake, and never actually come into play. Yes, deffkoptas have mob rule, but only because they're Orks, and all Orks have mob rule. They just happen to be the only Ork non-vehicle unit in the codex that cannot take advantage of the rule.
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