An authentication issue was addressed with improved state management. This issue is fixed in tvOS 15.5. A local user may be able to enable iCloud Photos without authentication.
[
{
"product": "tvOS",
"vendor": "Apple",
"versions": [
{
"lessThan": "15.5",
"status": "affected",
"version": "unspecified",
"versionType": "custom"
}
]
}
]