That statement reeks of a story made up after the fact. You can imagine them sitting around their police station after they let the kid go.
Police A: "Oh shit, this kid was just bringing in a clock to show his engineering teacher. There are several witnesses who heard him tell us it was just a clock. And now everyone in the world is watching. We're fucked."
Police B: "Nothing's fucked, bro. Be cool. Think. Think. Wait, he never told us whhhhy he brought the clock to school."
Police A: "Why does that matter? he told us it was just a clock and it is just a clock! Did we even ask why he brought it?"
Police B: "I didnt ask why. Did you? Even if you didnt, its still somethin' you could say."
Police A: "That is so stupid."
Police B: "Still, you know... it is somethin' you could say."
Police A: "That is so stupid. You are stupid. And, we are fucked!"
...later at press conference
clears throat
Police A: "...he told us it was a clock buuuuut, he never told us whhhhy he brought the clock..."
glances over at Police B who has the stupidest grin on his face