I think my E53 X5 just fell victim to the common expansion / coolant tank leak.

My question is that my car still has CPO warranty and when I called the stealership today they said it doesn't cover the replacement of the expansion / coolant tank? I thought a coolant leak is considered a pretty major thing so shouldn't it be covered by CPO?

Is the stealership lying to me and trying to make me pay for a repair that should be covered?

TIA