Earlier quoted context omitted.
That almost makes it worse—like they're vaguely aware that training too heavily on too small a data set makes badly trained models, but are unaware that it has a name and is an actual identified problem.
They've even described how they overfitted it! For five world cups, a simple model based on ranking and goal difference in the group stages[1] predicted it four times, so they invented a somewhat subjective variable on defensive strength to cover the teams that didn't score much... [1]yes, both of those are endogenous variables...
I appreciate everyone's feedback. This is a preprint, and I was looking exactly for this kind of constructive scrutiny to help improve the work. The data is limited, and I'm well aware of that. I just thought this would be a good first step, with encouraging results (and, after today's match, accurate ones too).