I think Andrew had already severed his official ties with Google when he started Coursera, since he was probably the person to recommend Google hire Geoff Hinton to replace him (see [1]). If Andrew had returned to Google, I don't think he would have as great of a leadership role as the one that Baidu is offering to him, since there are several other AI leaders that are just as prominent, if not more so, at Google (like Hinton and Kurzweil).