2001 Camry V6 supposed non-interference?

***** 2001 Camry v6 LE 3.0L 1MZ engine. Purchased vehicle to help someone out financially* knowing it had over 300k miles and eventually in need of a timing belt. The thing ran so smooth that I had several people ask me, "hey is that thing even running?" Due to the smooth running engine, my personal knowledge of the previous owner and his maintenance schedule, and the fact that I confirmed with Toyota dealership, several National engine rebuilders, and online sources* that this was a NON-INTERFERENCE engine, I decided to go ahead with the purchase regardless of whether the previous owner had a timing belt put in at the recommended intervals. He was 2nd owner and could not confirm if belt had been replaced.

***** I figured the worst that could happen is the car leave me stranded, and then I go put a new timing belt in. Just to confirm, I cracked open the timing cover and sure enough the belt was checkered but not frayed so I figured I've got a couple of months.

**** About a week before the incident, I started hearing non- metallic, sounds of rubber hitting the timing belt cover and that was my cue that the belt had started to give. I had already planned to schedule a time slot with my mechanic to get a belt kit installed including belt, idler, tensioner, new water pump, and Cam and crank seals but he was tied up, and I was tied up so I figured no harm, we'll do it in a couple of weeks. Finally, idled up to a parking spot, went and took care of business, came back 30 minutes later, and fired the engine up. It ran but very rough then died. Started it again, same thing. It would turn over and run but very rough and then die. I pretty much knew what had happened.

***** Towed vehicle to mechanic who finally squeezed it in a few weeks later and confirmed that it was the timing belt. He indicated that he was a little jammed up but he'd get to it in a week or two. Finally called me back a couple weeks later and said the engine is seized and piston is hitting one of the valves.

***** Checked back with some of my original sources and none of them can think of what would have caused this problem in a non-interference engine, however one guy did mention to check a valve spring to see if one had collapsed and allowed the valve to drop into the combustion chamber far enough to get hit by the piston. But he said he'd never experienced that personally. Mechanic is trustworthy as I've used him for many years, but just can't imagine what has got me into this situation. Any ideas appreciated.