This isn't complicated. All the DNA establishes is that Reyes penetrated her and ejaculated. There were undoubtedly DNA samples left by the others on her body but this was thirty years ago and cops did not think in the same terms they do today. Statements by the the five, some of which were prior to formal questioning by the cops, detailed some of the ways they sexually assaulted her and the violence they used against her, like the violence they used against some other victims that very day.
There is a burgeoning industry that sells one-sided "documentaries" about guilty-as-sin criminals to gullible, naive middle class white people and makes them believe all sorts of improbable things. Like with guilty-as-sin Steven Avery. Or guilty-as-sin Jeffrey MacDonald. And on and on. These filmmakers count on the fact that dopey viewers won't view their works with skeptical eyes. You'll notice that in all these cases, the viewers are asked to believe in large conspiracy theories to explain how these innocent criminals were railroaded. That should probably be a warning sigh to you.
There is one common denominator among inmates in our prison system: they're stupid. When these kids bragged of what they did on the way to the precinct station and in the station prior to being questioned, they demonstrated this. It's amusing to listen to the naive, gullible, dopey people to whom I initially referred, buy hook, line and sinker this idea that these kids just had no idea what to do once taken into police custody. By that point in their lives, they had many experiences lying to adults and authority figures. The problem for them was that the adults they were dealing with after arrest were not naive and gullible and dopey and they weren't smart enough to figure out how to avoid incriminating themselves.