Its rather apparent. He obviously knows that its more like 20-30 million already. He is just using the Liberals and RINOs own admitted numbers so that he can focus on the real issues of policy difference
Emphasizing the dispute over the actual scope of the millions of invaders distracts from the short sound-bite of time he has to get his points on policy out.
In other words...Duncan is being an effective campaigner.
Something the RINOs try and deny.
I suppose it's as plausible as any other.