According to multiple sources, the uranium of interest is under 500 kgs and easily transported using relatively light motor vehicles. Ted Postol of MIT has done multiple interviews explaining how it would have been managed.

Iran could have easily scattered the material over multiple locations well before the US/Israel hit the site.

Having said that, I find it incomprehensible that if Iran had the material, they didn't finish the task of building a working weapon. If you have resources the US wants to steal, and you have no nukes, you get attacked.