You're confusing ULAs (Unique Local Addresses) with LLAs (Link-Local Addresses).

(ULAs don't need the interface specified.)

ULA: fc..:… and fd..:…

LLA: fe80:…

[ed.: By the way, sin6_scope_id is where the interface identifier is stored in struct sockaddr_in6. So, basically every single IPv6 address object you're handling has the field for it.]