PVC is impermeable to water, so the leakage was due to the “sealed” end caps. However they were sealed it wasn’t good enough, and I would suspect seasonal temperature variations contributed a lot as the pipe and caps would expand/contract repeatedly.
Maybe we should take a lesson from the ancients. We still find sealed clay pottery jars in the bottom of the Mediterranean that haven’t been compromised thousands of years later. The Greeks and Romans sealed those with ceramic caps and tree resin/pitch, possibly with wax, lime or other simple compounds added.
From what I understand the wine or whatever in those recovered containers is not drinkable. Even though “sealed” and what was inside may have “looked” like wine it was contaminated. Given enough time I suspect nothing is completely impervious. I was surprised to learn from my sailing friend that fiberglass and gel coat were not impervious.