I think he (Thurman) saw no other option for his plan to succeed. The "nuclear waste containment facility" ruse wasn't going to hold up to scrutiny indefinitely—and those silos were
never going to be voluntarily populated once that particular cat was out of the bag (if ever, for that matter).
Without the convention and the immediate, local threat, there would have been resistance—questions. And empty (at least partially) silos. They needed a "flood" to convince everyone to get on the "ark." Nothing else would have worked.
Besides... if a large contingent of the population were left
outside the silos, they'd have destroyed the silos trying to get
in once they figured out what was in store for them. It would have been too dangerous to Thurman's new "Order" to leave civilization intact outside of the "silos" for even a short period of time.
Just my two cents. The logic (however megalomaniacal and insidious) seemed consistent to me.
