I think it's more that "hey you might assume that ships are these big strong things that you can't just cut through but that's a misconception; they're actually weak enough they can't even support their own weight -- a fact that was exploited by torpedo designers."
It's also not true for some ships, depending on exactly what you're talking about.
It isn't just being forced to support its own weight by a bubble that sinks ships... it's the force applied in the direction it's applied by the explosion of the torpedo. Most hulls aren't designed to handle a sudden non uniform upward moving force in their middle accompanied by a shock wave.
