This isn’t a pin because it’s not taking in a name as an argument. It’s taking FBoneReference which has no blueprint presence. You see the bone name as a convenience to the end user when selecting a bone, but it’s mostly using the bone index.
If a name or index were passed in, the engine would need to resolve that name or index to an existing FBoneReference each frame since pins can have variables that can change at runtime, pretty much destroying performance.
I’m not saying there’s no way something like this can’t be done with good performance, but this is why it is the way it is. Frustrating as it may be!
This is why we need blueprint constness! Hint hint core engine devs