My views:
Low MAF when tested at WOT and trying to get the rpm and MAF up high can mean a blocked exhaust, especially if there's no other apparent reason. In logs I've seen here that turned out to be blocked cats, the rpm might go up fairly high, but not as high as it normally could, and the MAF, instead of increasing along with the rpm, levels off at some intermediate level. The engine is spinning but isn't pumping as much air through as it should.
I've read of cases where the cat core overheated, melted, and broke loose. But I don't recall cases where the core separated from the housing, moved back, and blocked the outlet, without it having first sustained some form of damage.
The tech identified the cause of that symptom but perhaps didn't address the reason the cat failed in the first place.
If the cat had overheated, then that was more likely caused by an external problem, e.g., too rich mixture, misfirings etc.
I presume the quotes relate to the original cat that was replaced at 100k miles. Now the car has 136k miles and, apparently, the new cat has failed. But what are the symptoms now and what diagnosis was done to conclude that the cat is bad? If the symptoms and failure of the new cat are the same as on the original, it's possible that the cause of the first cat's failure was not addressed at that time, and its continuing presence contributed to the new cat failure. But this is speculative; there isn't enough information, and I doubt that at this time we could find out what other tests the first tech might have done to eliminate outside factors.
