If the _id field is not the shard key or the prefix of the shard key, _id index only enforces the uniqueness constraint per shard and not across shards.
If the _id field is not the shard key nor the prefix of the shard key, MongoDB expects applications to enforce the uniqueness of the _id values across the shards.
There is no contradiction between the two. They both say that if you must have uniqueness in _id field using default MongoDB generation it must be a prefix of the shard key. Otherwise there is possible duplicates.
If your application does not care about the _id field it is not significant for you.