First, sorry see your riding stead out of action.
If they put a 15 year warranty, then surely they have decided their mechanical design, material choice and manufacturing processes will survive - at a minimum - that stated usefull life expectancy for it's intended purpose.
This simply couldn't exclude the accumulative effects of normal wear and tear on the final product for that period. It's a damn MTB frame, not a stationary ornament and not a moving part like a chain!
This invisible wear and tear hasn't been measured, and so it's left to the Management Team to decide (!) the circumstances of failure in that period and open a nice suitable door out of the warranty clause.
It failed within the warranty period, so you have a case. If there is no stated weight limits, or it said "not to be thrashed day in and day out" or other conditions, then that's Litespeeds problem not yours.
A written 2nd opinion from an engineering authority would probably serve the case better to describe the fault. It's a crack - I don't see anything worn out I just see a manufacturing fault in the tubing.
Litespeed see it's worn out......but are not able to show proof.