Yes it is BS unless that gravel got into the pulleys where it will destroy a belt in no time flat.
A belt will tell you why it failed by looking at it.
Something was wrong with either the mower or it's operation for a belt to fail at 7 hours .
Do you still have it ?
if so some photos of the damaged areas of the belt will allow at least a determination of the mode of failure.
In any case it will be a moot point as blades & belts are not warranty items.
Picking up at.dealership in morning they wanted 149.00 to repair not gonna happen.they belt had a little tear or notch in front left side when I went to mow sunday evening it wouldnt engage I put it on with 1 finger I supect.lowes had kids put together and installed wrong does it need to be tightened? I had to adjust deck height when I got it when I had it in 1st.slot it measured 3 inchs high.