I don’t know enough about bombs to say something definitive, but it seemed to me that the bomb itself didn’t detonate. It just fell from a plane and caused a gas explosion.
Of course since I don’t know I might be wrong and the article might be right, but the explosion seemed too small for a 1500kg bomb.
1500kg bomb on a delayed fuse detonating under the city street provides a lot of oomph, but in a confined area.
Just one man’s opinion, of course.
Guessing it was designed to penetrate the earth before detonation, not on the surface.
But 18 seconds would be a very long delay?