score:3
On the SAS website (that you linked to), if you click the link 'checked baggage' (https://www.flysas.com/gb-en/travel-info/baggage/checked/), you get to a site where it says: "Checked baggage is not included in SAS Go Light, but it can be added for a fee". You can buy it right there.
I would not trust offers from third parties, but it could be they sell you a bundled ticket price with luggage. It could also be that the fine print reads on page 473 'the details of the allowed luggage are defined by the airline", and you are out of luck.