An open mould is a mould that is not proprietary to a single brand offered by a manufacturer to anyone (minium order quantity taken into account of course). Basically you buy frames and you don't have to handle anything related to the mould itself since it's owned by the OEM. As a buyer you can always make a contract that they supplier needs to guarantee to have X units of frames produced for you over Y amount of time to guarantee your product continuity.
For closed moulds, the design and moulds are owned by the brand (sidenote: the carbon lay-up is often arranged by the OEM with some feedback loops based on prototypes and the brands have no idea about the exact recipe). There are multiple ways those brands pay for that mould, i.e. pay everything upfront, pay overtime, pay the first frames with an extra fee to pay-off the mould.... many procurement options are feasible.
Another option is that a brand buys an open mould and makes it proprietary. Less common: a brand might sell his design/mould after moving to a new design/product.
Back to topic: With the volumes Decathlon sells, the moulds are paid off very fast and as mentioned above, it doesn't make sense for their business to risk open moulds.