Yes...but he's not making the claim that "100% of foster kids who get (some small number of adults caring about them) will be successful", he's saying that "the minimum I've observed for foster kids to be successful is (some small number of adults caring about them)".
The latter claim is perfectly valid to make when 100% of survivors have trait X; you are correct in implying that it would be invalid to say having trait X will guarantee a survivor.
The latter claim is perfectly valid to make when 100% of survivors have trait X; you are correct in implying that it would be invalid to say having trait X will guarantee a survivor.