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I’m an insurance investigator.
I do not believe it was their responsibility to check, and I certainly wouldn’t authorise a liability claim against them if I was provided with this information.
I think I would graciously accept what they have offered you and understand it to be ex gratia (good will) and thank them accordingly.
Pipes absolutely can loosen over time, especially if they weren’t tightened sufficiently in the beginning.
The flex of the floor/bath/movement of water through pipes etc certainly can add to this happening. They cannot be expected to check what they were not tasked to deal with, because realistically, where does this stop? If they touched a pipe that was not their responsibility, that was then connected to another poor joint, then they may be held liable. They’d have to check everything to ensure there would be no further issues and this is just not feasible