That statement does not even look logical at a quick glance.
Reads to me like someone jumping on a trend, with how the current market is looking, and the wider issue of a global recession, people will be believing this for a while, and that might increase sales for their books.
I wouldn't look at this statement, and expect a sound, logicically approach. It's simply a marketing scheme, which will likely work. Doesn't matter if they're wrong in the long term, people would've already forgot. However, when you look at it short term, people are going to be like; "they were right!", and look at his work. Since, it's very probable that if we hit $100k tomorrow, we'd have a price correction not long after that, and then they'd be able to bask in that glory for a little while.
To be honest, I'm not overly familiar with their work, but if you're going to make bold predictions at least go into depth on why you think so.
Overall, nothing statement which likely has an agenda attached to it. Also, ignore people's titles, mathematician doesn't really make his reasoning any more sound. Judge people on their logic, not their titles.