Agreed. It'd have to be something else to force the Zhodani to venture that far out of their comfort zone and into Imperial space.So why not send a group of super spies after it? You know, PCs...
Or jump through empty hexes with their entire navy to Rhylanor.
The super McGuffin is a pretty lame excuse for the FFW.
Let's keep "mountain-sized Ancient alien psionic technology to stop a weird wave of energy expanding throughout the galaxy" and make their expansion more plausible, such as economics (Zhodani economics based on feudal technocracy just don't work, and what's happening to them now will happen to the Imperium in a generation), necessity (Consulate worlds on the Zhodani frontier are collapsing, and they need material resources from Imperial worlds in the Marches) or the ambitions of a single individual Noble who fancies himself as a Zhodani Alexander the Great.