An easy willingness to tell funny stories or jokes about oneself is the surest mark of supreme self-confidence.
Statements
Causal-ish The clearest indicator that you're supremely self-confident is if you're very willing to tell funny stories or jokes about yourself.
Comparison This willingness to poke fun at yourself is a clearer indicator of self-confidence than is your willingness to have others poke fun at you.
Evaluate
The second sentence is barely telling us anything new. After all, once we say "the surest indicator is X", then by definition "X is more revealing than (anything else)".
Since the clearest indicator of self confidence is poking fun at yourself, then by definition
The fact that people often don't acquire this willingness until late in life is a very wishy-washy quantifier. "Often" is similar to "Many". There are definitely at least a handful of cases, but it doesn't mean that this is necessarily true in most cases.
We could combine the first sentence with the "not acquired until late in life" and derive something like, .
Goal
Hard to say what the correct answer will be saying, but it seems likely that it will make use of the very strong claim in the first sentence.