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.
"Having understood name-and-form, which is a product of prolificity,
And which is the root of all malady within and without,
He is released from bondage to the root of all maladies,
That Such-like-one is truly known as 'the one who has understood'." (Snp 3.6)
"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)