Okay, what about kneeling to propose to a partner... Is that one person submitting to the other?
Yes, it is.
The point here (and always) is the context in which the kneeling is done. Kneeling in this context by police and random people is clearly a submission to and acceptance of the ideology that "black lives matter". Why would anyone ever have to kneel down to accept that "black lives matter", or any other self-evident statement? Do you accept that the sky is blue on a cloudless day? If so, you must prove it by kneeling down in front of me. Does that make sense?
If someone asks me if I agree that black lives matter, then I'll say yes or no, I won't kneel. Why would I? I'd say if you asked any normal person on the planet if the lives of other human being matter, they'll invariably say yes. Now, they might not show that in their day to day lives, but that's human nature, and the reasons for it are many and varied. People who don't offer a reasonable amount of respect to other people will generally be socially ostracized in some way, and if they take it too far, prosecuted under existing laws that forbid such behavior.
Of course, some agents of the state who have a prejudice against some section of the citizenry that they perceive as being different to them or less than them, will exploit their power to abuse those citizens. That happens a lot in many different ways, but that is not a social issue, and should not be interpreted as one as is happening right now) but an issue that relates to the relationship between people and the forces of law and order of the state, and historically, it is difficult for citizens to have their greivances in this regard addressed, for obvious reasons.
Every time I see someone kneeling in this context I think of this, because I think that the ideology that is acting through people who demand the kneeling has its source in something rather nefarious.