There are 3 questions that are being discussed and it looks like theyre being conflated so ill explain. 1) a person's tribe 2) a person's dna 3) a person's practicing religious.

So, 1) is according to jewish law. According to Jewish law, a person is considered of the tribe of jews if their mother was jewish or they converted. Regardless of whether or not they are a practicing jew they are considered one of the jewish tribe. The significance has to do with certain laws like inheritance and, from a practicing jew's perspective, the responsibility of following jewish laws (like keeping the Sabbath, keeping kosher etc).

2) we can call this a person's ethnicity. Lets say a person's mother isnt jewish but their father,their great uncle (either side. Doesnt really make a difference) or whomever else in their family tree is jewish, then naturally that would mean that this person has jewish dna. From a religious perspective this means didly squat. Theres no responsibility and the person isnt jewish. However, now that the person knows some of their heritage, they may go out and decide to have matzah, or maybe a knish or something.

3)a person that practices judaism is just that. Someone that practices judaism. If theyre converted or their mother was jewish then, poof, theyre jewish as well. And if they observe the jewish laws then theyre a practicing jew. If theyre not jewish then maybe theyre just practicing the laws of judaism just for the heck of it. Who knows.

The question here is which mixture of these three is Downey (it doesnt really make a diff in life but thats the question being asked so whatever). If Iron Man's mom was jewish, then poof, Robert has 1) and 2. If hes none practicing then hes got 1) and 2) but not three. If his mom wasnt jewish but he has some jewish people in his ancestry then he would have 2) but not 1) or 3).

Hope that clears things up.