Note the odd scalloped pattern - the little wave shapes between the torn rivet holes. Have you ever seen that pattern in any other wreck? I have, but only once. I can show you that exact pattern of failure where two skins were lapped and stitched with a double row of rivets, and it's very apparent why it failed the way it did.
It looks like a tensile failure within the sheet. Specifically, it looks like the keel was fixed (in the stationary, not repaired, sense) and that the sheet was pulled laterally away from it (to starboard). The fracture started at the front and travelled from a rivet hole back into the piece until it hit another hole or the perpendicular stresses exceeded the parallel ones and the fracture line changed direction accordingly, zigging back outward in the direction of the pull.
Try pulling a piece of newsprint apart (top half from bottom half). I bet you'll get some tears that are nearly right angled at the tip, similar to the sawtooth pattern at the back of the artifact. Very different from the fatigue-looking (piecewise linear) failures on the forward and aft edges.
Given a certain combination of aircraft position, surf conditions and timing, a wave coming through an open door and impacting the interior of section 269 might load that area in such a way as to produce high tensile loads in the skin. Enough to yank it away from the double row of #5 rivets that should have been there holding it to the keel? Maybe.
There should have been additional material both forward and aft of the artifact for it to have reached the formers where it would have been attached. The stringers in this area seem to have passed through the formers intact, so the repair skin might have extended forward to the next former, but the skin went to .025 thickness (as built) aft of 293, so if the repair matched the existing construction, there would only have been a couple inches of new skin aft of the tear.
Perhaps that section of the airframe was split in half early on and the artifact was then beaten out of the remains of the starboard side over time. That gives you a tensile fracture on the keel edge and fatigue on the remainder.
I'm still skeptical about the heads being blown off the #3 rivets without distorting the holes, but I'm more skeptical about someone removing the rivets afterward without more evidence of tool scratches or other damage. The first one is easy to test, the second is akin to proving a negative.