I'm not sure what the under-40 part is, other than a cut-off point - it's not really important, other than being younger rather than older.
From a cursory read, it posits a theory that anyone whose white blood cells of the immune system beats off the virus quickly and thus the body getting round to making antibodies. What that might also mean is that younger recoverees may not have antibodies in their plasma.
This is more of a testing study to find possible leads for future work, rather than a plain failure with no benefits coming from it.