Well, it doesn't have to be downwind, but certainly would be much better if it were.
Most backyarders don't have the space to do a full and true biological quarantine....
...or the awareness and skill to avoid cross contamination via clothing and equipment.
Did you read the article?
If this...
Maybe not 4/10 for one bird, but distance is important...down wind if possible.
It's not just about distance, but also how you move between the 2 groups.
Here's a great article on quarantine:
BYC Medical Quarantine Article