No, you know better. I'm saying it's possible Mark wrote a first copy, then later wrote another version with a different ending, perhaps after getting more information from someone he felt was reliable.
The earlier manuscripts like Sinaiticus and Vaticanus are copies made from Mark's first version. The later manuscripts are copies made from his second version.
All this is speculative and no one really knows the answer. Personally, I believe the longer ending should be approached with some skepticism; but we can't rule out that it's legit.