I also thought it was weird, so I did a bit more digging. Looks like the initiative started in 2024 (with help from the Lyzi platform, which instantly converts BTC to EUR), and by the end of 2025 they started accepting the Lightning Network.

https://www.bougerabordeaux.com/actu/gironde-un-supermarche-recompense-les-paiements-en-bitcoin-avec-une-grosse-remise/