It has potential to be true. It was always weird that the Gorosei would allow for a Yonko to waltz into their chambers, and not even be the slightest bit on guard about it.
And the way they talk is if the meeting was requested through a third party, which makes it even more suspicious, as Shanks would have had to contact someone on Mariejois, to get in contact with the Gorosei for him, to even request the meeting. The kind of defeats the whole purpose of being secretive about it.
My take is that the Holy Knights all come from non Celestial Dragon, prestigious families. "The Fearland Family" is one of these families of Holy Knights. Perhaps Shanks was meant to one day grow up to be a Holy Knight, but God Valley change all of that. If that's the case, then someone else from the Fearland family, would have taken Shanks spot as the Holy Knight. A twin brother, as cliche as it is, wouldn't be a bad idea.