I wonder why he didn't argue against the point by using an actual billionaire to illustrate. Instead he chose someone who is not a billionaire, and imagined them becoming one with nine and a half months of constant 93% growth. Couldn't his counterargument become stronger without the underpants gnome logic?
He is probably the actual billionaire in question, but doesn't want to highlight that point given the populist backlash against billionaires.