I think the Associated Press is extrapolating into tomorrow. It is realistic, not completely tentative. I didn't find the exact article stating Hillary Clinton has secured the Democratic nomination, but, it does make sense. To state she is out of reach of the nomination is not realistic. She will win enough delegates tomorrow to cross the finish line. Not to say Bernie Sanders will have a poor showing, that is not expected, but, the reality is Hillary Clinton will finish her run tomorrow.

The Associated Press is probably looking at polls and they show enough voters lining up with her nomination.