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...