The belt change interval is indeed 150K miles. Schedule is available @ https://volvo.custhelp.com/app/manuals/OwnersManual/om_id/149
I suspect that they get away with the extended interval because of the clutch system. It reduces the tension required and changes in load. It's a short belt too.

I inspected it looking for delamination that would cause a "knocking" noise. Found nothing out of the ordinary.

I would love for the problem to be a faulty alternator or other accessory. Unfortunately my mechanic diagnosed it as a problem internal to the engine. So it sits waiting for the dealer who sold it to us to come tow it back to the dealership for further diagnosis. He and I are both dumbfounded as to what could go wrong at such an early age with no warning - and no warning lights.

We shall see. In the meantime it's a drag to be paying for a car we can't drive, but happy that the problem happened within the short warranty period.