In their defense, Masayoshi Son made much of his fortune by applying the greater fool theory (and lost much of it that way as well). There's a chance they really just took a calculated risk here, and with many of their other investments too. The risk being not so much the success of the company like normal VC investments, but explicitly a "maybe we can profitably push this through an IPO, let's see if the sheeple fall for it" kind of thing.
Hypothetically, if he was really good at this he'd pick a highly leveraged market segment, attempt to crash it with a ridiculously implausible high-profile failure at the tail end of a bubble, and benefit by shorting everything that wasn't nailed to the floor.
Or maybe he's just a bit gullible. It's hard to be sure.
