Well, neither him nor I are a metallurgist, he's basing his opinion on his experience and that of his customers. Perhaps I should pick his brain on how he backs up his claim.
Specifically, he told me that the output shaft is similar in size to the pinion shaft on the dana 44's that are typically behind them, yet it's always the d300 shaft that breaks first.
So, in the absence of scientific training, I'm going by anecdotal experience of people who've been there and done that.
And no, he doesn't try to sell me anything. He builds very custom rigs, really cool stuff, and I can barely afford to buy the parts to build the rig myself. I call him my machinist, but perhaps wheeling buddy would be a better term. I've seen the work he does, and it comes out really nice. I use his work as a benchmark against which to judge my work.