Our chat experts can handle your question in more depth - but from the documentation end, I just wanted to apologize for the confusion and agree with you that the doc is, in fact, unclear about that point. I will emend it today.
The very short version answer is that you can chat with any community member - friend or not. Your friends are shown in a stacked list with their presence indicators on or off (green or grey) above the search field. You can use the search field (as you saw) to find any member of the community and initiate a chat conversation with them.