There are different ways in which this situation could be interpreted. I have heard the following positions argued at various points in time (none of which I am arguing for or against, by the way)...
* The listeners already had strong sila (morality) or bhavana (mental cultivation) beforehand, and not-self was just like adding the missing piece of the puzzle
* Stream-entry is easier to achieve than later Buddhism would have us believe
* Stream-entry is more about having the Right View that leads us to nibbana, than it is some kind of meditative attainment
* The Buddha tailored his presentations to accommodate the learning potentiality of his audience
Take or leave them as you see fit, and don't shoot the messenger.
“I hope, Anuruddha, that you are all living in concord, with mutual appreciation, without disputing, blending like milk and water, viewing each other with kindly eyes.” (MN 31)
What is the final conviction that comes when radical attention is razor-edge sharp? That the object of the mind is mind-made (manomaya). (Ven. Ñāṇananda)
"If you have always believed that everyone should play by the same rules and be judged by the same standards, that would have gotten you labeled a radical 50 years ago, a liberal 25 years ago and a racist today." (Thomas Sowell)