A betrayal to the fandom is irrelevant though lol, there are figures in many stories that betray the audience, its literally their point of character.
Yes, Shanks has been altruistic 99% of his appearances, but the key point about Chapter 907 is the fact that he (at the time when not assuming he had a twin) did something shocking to betray the reader. Its why so many people were in denial about his behavior anyway at that point.
This also doesnt really change the fact that he still reacts oddly to Luffy in 903 and deciding to go after One Piece in 1054. It also doesnt change his brutal nature (which yes, is justified) to Kid and Bartolomeo. The pure fact the latter was attacked, at least shows Shanks' crew isnt afraid of attacking Luffy and his fleet. These things still stand even if Shanks has a twin God Knight brother.