Either driving through deep water or if the car was ever in a flash flood (where water suddenly covered the air cleaner, which I've seen btw) OR if an injector stuck open (allowing gas to fill a cylinder with gas) would all be likely causes of why an engine would hydrolock and bend a connecting rod.

Did you buy the car new or used? If used, you wouldn't know what might have happened before but if it's a noise from a bent connecting rod, it should have been noticeable from day 1 I'd think (the day it became bent) and constant, not something that's been just recently noticed and only when cold.

The story is strange because you recently posted it as being diagnosed elsewhere as noisy lifters when first started. Damaged, bent engine parts are typically always noisy (cold and hot).

If the connecting rods didn't bend from being previously hydrolocked, someone who denies you extended warranty coverage then needs to explain to you why it bent otherwise because they don't just bend from any type of driving otherwise.