Looks like NWC integration of BTCpay lets you connect to a node with NWC - not connect apps to your BTCpay.
Meaning, you can paste NWC string into your BTCpay under 'custom node' to use this node in BTCpay. You can generate them with wallets/nodes supporting Nostr Wallet Connect, like Alby, Umbrel, Start9, Lifpay. Same applies to Amethyst