Recognition of Kuffar being obligatory is not about things you do not know.
It is not about being obligatory to investigate to find out if someone is a believer or not.
Rather, it is about things you do know. If you know something about a person for sure that is clear disbelief but still believe he is not a disbeliever, that is what it is about.
Meaning: If you know that someone is a Christian, you cannot doubt his kufr then. Because it is known for sure that Christians are confirmed kuffar.
If you don't know anything about a person that makes it clear to you his beliefs, why would anything be obligatory on you?
Use a bit of logic as well. How can it be obligatory for someone to know something they do not know? If you see someone random in a crowd, how are you supposed to determine if they're Muslim?