A portion of the uranium in a reactor core turns into plutonium. It has to be at a high enough level to make isolation practical. Also, about 25 lbs is needed to make a bomb.
I guess the Iranians calculated the reactor core had cooked enough plutonium to be usable.
Indeed, the fuel rods they removed contain the plutonium. Filling the containment vessel with concrete is a meaning less gesture. Where they removed the core to is the question.
Removing the core means they are in the final stages of bomb construction.