First of all, I'm a Risk Analyst... That does not imply that I'm specialised in Credit Risk, more specifically in Credit Risk Modelling (As you are referring to Champion / challenger models). Lucky for me I've sat in on some Credit Risk Modelling Committees and understand a little.
The champion model is an existing credit risk model (such as the behavioural credit risk model Afterpay is using). When this model shows signs of deterioration it gets recalibrated, but sometimes it's worth creating a new model (challenger).
What is really making me scratch my head is your "assumption" that Afterpay doesn't have a quantitative credit risk model (or simply a "champion" model). The fact that you simply call it a champion model tells me a lot about what you actually know about this space.
What relevance does any of this have with my previous post? Zero.
Just big words. Smoke and mirrors.
I guess your strategy is, when called out try and evade. Smoke and mirrors.... Unfortunately for you I see through it.
