That phrase is interesting because it's imprecise enough to be true and false at the same time.
We've not specified what the proportionality is to. We've also not really specified what a "victim" is. Does that mean an unarmed person? A person that didn't deserve it?
We know black people are killed by police disproportionally to white people. This is math. Unless the numbers are more than 3 to 1 it would be disproportionate to general population. Now one might have to argue we'd have to go into geographic factors. And we'd have to parse through different things.
We also know there's going to be a certain thumb on the scale in all the retellings because there simply isn't much robust oversight. The people shot are dead. The people shooting are the ones often writing the reports. The people overseeing them are most often strongly empathizing with them and very often willing to cover for them. Our data is most often rosier than reality, logic should tell us.
Does this mean these killings are happing all over? Most likely not. They are certianly not happening at the rate that certain hysterias seem to paint. But I do think it's a thing (and you know what, police killing white victims is pretty terrible too)
Just taking the raw numbers, police shoot and kill twice as many whites as blacks. As you correctly note, if all other factors were equal, the ratio should be on the order of 3:1.
But all other factors are not equal. Blacks commit more violent crimes than whites do. I've seen summaries of two different surveys--one a couple of years ago and one a couple of days ago in the
WaPo, that say that, under the same circumstances, police are very slightly more likely to kill white suspects. When you get to other forms of hostile interaction, the statistics all support the idea of cops harassing, hassling, getting physical with blacks they encounter in the line of duty than with whites. And all that stuff rightly leads to blacks not trusting cops, and thinking that "call the police if you're in trouble" is crazy talk. But cops are not killing blacks in circumstances where they would not kill whites.
The idea that police "overkill" young black men is widespread, particularly among blacks. I listened to a podcast today on which the guest speaker was Michael Steele, the former head of the RNC, African American, and he talked about how he felt compelled to warn his sons of how to behave around police officer because "we know" how dangerous it is to be black and interacting with police. But he's mistaken. It's just not true.
It also was not true, BTW, that blacks were killed in disproportionate numbers in Vietnam, although many black leaders, including MLK, and many others as well, believed it at the time and still do.