My general understanding is that dealer price from supplier is approx half what dealer charges consumer. Usually. Of course dealer has overheads but basically selling a bike has a massive profit margin. Its great that decathlon choose to absorb some of that.
The vast majority of the sales pitch around frame design is just marketing- a one off cost that pays dividends across a model or range.
Ditto branding. Once you've secured a brand design its cheap to carry it across shape, size and colourway. If you sell a few units.
Meanwhile there are plenty of us who'll keep buying inappropriately expensive shoot that is the same shoot that decathlon sells for much less.
The phenomenon isn't restricted to bikes.