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.
"Do not force others, including children, by any means whatsoever, to adopt your views, whether by authority, threat, money, propaganda, or even education." - Ven. Thich Nhat Hanh
"Whether I were to preach in brief, Sāriputta, or whether I were to preach in detail, Sāriputta, or whether I were to preach both in brief or in detail, Sāriputta, rare are those who understand." (A I 333, Sāriputtasutta)