So, OP knows they hit something, knew it was leaking (may not have known what they hit, but knew they hit "something") probably continued home and parked. The "didnt realize it" can be partially true as maybe they didnt know what they hit, but they know they hit something.
Nobody said that it was leaking outside of the pack, so how do you know "he knew it was leaking"? My reading is that the leak was internal to the pack, and it was obviously a small enough leak that it hadn't triggered the coolant low warning. We don't know how long the pack was damaged before the internal coolant leak caused the failure... (For all we know it could have been damaged before he bought the vehicle.)