It's true that it seems unlikely that all 4 bombs would fail to detonate. But it would make sense if they were made at the same time and the failure was due to some time related deterioration of material.
So, in any event, they don't seem to think it is a hoax/joke. It is scary that someone pulled this off, that may be the actual "message".
Bad mixture to begin with?
"It is scary that someone pulled this off, that may be the actual "message".
Or due to faulty preparation of a single batch of the material.