CX-5 bolt gave out 8 days after oil change and inspection at Mazda, causing catastrophic engine damage

They said it wasn’t due to the mechanics making an error, but how do I know that?

It was 8 days after an oil change and full inspection where they were touching the engine and they didn’t say anything about it.

I looked it up and it’s commonly caused by technician error during the oil change.

Mazda corporate agreed to pay $2,500 but that’s only a third of the cost

How do I know if it’s due to the mechanics and if their insurance should pay?

It’s a 2017 CX-5 GT for context

Thank you so much!