So after some sleuthing, according to this post, the offer does in fact NOT show upfront as being applied when using the app nor at initial entering of phone/alt ID at the pump, but only after swiping/tapping a payment (or confirming payment in the app).
https://www.doctorofcredit.com/shell-gas-fuel-rewards-22-gallon-discount-on-nascar-race-days-37-races/
So a great big pfft to Shell for not explicitly stating that their app/system is stupid and that the offer does work, you just won’t see it until after the fact. I guess I’ll try again next Sunday when the next round becomes active.