I think symbolic gestures are important, and they can change and develop. The game of thrones reference was naff, but I don’t think Rabb is wrong to reference the history of kneeling as subservience, but the gesture today in this context is different from what it was (& the GoT way in which it reflects servitude).
Kapernick’s knee was - at least to me - powerful symbolically to me because I felt he was trying to find a way to demonstrate the injustice he rightly felt in a way that also respected the American flag. Kneeling is a time-honoured gesture of respect (and indeed also subservience). He was shamefully treated over it.
Maybe Kapernick also meant it also to reflect the abusive way in which policeman subjugated black men - I don’t know - but the contrast between him kneeling in rightful protest then, and the US cop’s horrific kneeling which killed George Floyd now in my mind further transforms what that gesture is and what it stands for as a symbol of public protest currently.
But I agree with others too that as a symbol, because of mixed cultural history and context (one kneels before God to pray/take communion, one may kneel before the Queen, etc), and because kneeling is subservient - if you were kneeling before a monarch in the middle ages they could swipe your head off rather than knighting you! - that the gesture may be less powerful in the future than a closed fist, for example.
But right now the gesture is powerful and important to black communities, and that matters; and white people kneeling (indeed everyone kneeling!) helps visually demonstrate to black communities a statement that they do matter - partly because of the subservience, actually. I’d kneel now, for all I dislike the gesture in its historical context - because the meaning of the gesture matters more than that history right now.