The 2.5% net profit margin is an average across all shoppers. It does not mean every single shopper actually makes exactly 2.5c profit for Coles per dollar spent. Some make less than 2.5% profit for Coles, some more.
Now, I'm not a retail expert so I don't know COL's actual sales strategy here, but you can easily come up with plausible scenarios to explain how they can keep giving you 10% off total shop and still make a 2.5% profit margin overall. For example, if 95% of shoppers don't use Flybuys email vouchers and make COL a 3% profit margin, and 4% use Flybuy discounts occasionally and make COL a 1% profit margin, and 1% use it frequently, and make COL a negative 2% profit margin, then you'd end up with an average margin of around 2.5%-3%.
Again, these are not COL's actual numbers, but they illustrate a plausible way to me to explain your 10% discounts.
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