http://breezewiki.vernccvbvyi5qhfzyqengccj7lkove6bjot2xhh5kajhwvidqafczrad.onion/minecraft/wiki/Talk:Explosion
As it is, how I understand it, the first one means if the first attenuation step doesn’t reduce intensity to 0, but the second step does, the block doesn’t explode, but I think this is wrong and also the second paragraph says otherwise (the first step has to reduce it to 0).