Rabbi Zechariah ben Hatzav

Good question. It is possible that the statement is directed at the public, but for him it is truly permissible. There is still room to be stricter on himself so that the public does not make a mistake and allow the 3rd.
Although the Gemara’s difficulty there implies that it is from the law, but what is harder than a divorced woman? Perhaps because he himself was being harsh and making it seem like she was forbidden, they understood that this is how one should act toward someone who is forbidden, and therefore they made it harder than a divorced woman.
Although the Poskim present this as a settled case law. In Beit Shmuel and Be’er Hetvet, they wrote that if only he knows about her and knows that she has not become impure, then it is permissible for her. This means that this is truly the gist of the law and not a special twist that he practiced on his own. But the law here is only out of fear of the conclusions that the public will draw, and not a real law, and therefore if they do not know about it, then it is permissible. And why in Fatsha there is a reason that perhaps his eyes have set on her because she is his beloved wife (meaning that this is a special thing in a husband, and in this he is different from one who is married to one of his wives). According to this, it is forbidden even if only he knows about it.