User blog comment:Shadow Archon/Hypothesis: Explaining away the Zerus Discrepancy/@comment-4817-20150513233805

Mengsk's communication device was found in or near the K-Sector, probably Char. There's nothing to indicate the Dominion or the Khala can communicate that far.

I don't think everything on Zerus is zerg. The plant life probably isn't, and there are critters there too.