It's pretty clear whan a rear main goes...there usually isn't any doubt and they'd be idiots to be wrong about it.

How many miles on your car?

How frequently did you change the oil?

What kind of oil were you using, and what viscosity?

I understand why you're incredulous, because most of us do something else to the car before we blow a main seal, and the main seals are usually only replaced as part of a buildout that was occasioned by another type of failure. However, it's not unheard of.

In my case, I burned a piston at Summit Point at 79,000 miles.