They might have reimplemented it (or not). I very much doubt that they would have called up Arc90 and asked them how much money they want.
(My own little theory is that one or a few Safari developers were avid Readability users. They saw the kind of license Readability used and noticed that all they had to do to make it a Safari feature is to give it a nice UI. And that’s when they noticed that they could easily convince others [superiors, that is] that it would be a worthy addition. A quite boring theory since it doesn’t include any grand conspiracy to bully Google, I know.)