There is no definitive difference between cordial and syrup. Cordial, perhaps, implies that it will be diluted and served as a drink, while syrup suggests a wider use for poaching fruit, drizzling on cakes, or adding to dressings. Both are made with sugar, water and flavourings – in this case, edible flowers.
There are a few ways to extract the flower flavours for syrup, but the primary difference is when you add the sugar. My preferred method is to add the sugar at the end of the process, once the flowers are