It means you are innumerate.
Although the math doesn't require an absolute population value, it's a bit easier to reason about it if one is used, so I'll round the population of the country to 300 million.
If whites are 70% of this population and 500 whites per year are murdered by blacks, then the white population is 210 million, and the rate of whites murdered by blacks is 500 per 210 million -- about 2.4 whites per million are murdered each year by blacks.
If blacks are 13% of this population, and 200 blacks per year are murdered by whites, then the black population is 39 million, and the rate of blacks murdered by whites is 200 per 39 million -- about 5.1 blacks per million are murdered each year by whites.
Ergo, as I said, these numbers, if believed, show that a black person is twice as likely to be killed by a white person as a white person is to be killed by a black person.
Well, only approximately, since 5.1 is not exactly twice 2.4, but that wasn't your quibble now, was it?
As other commenters have pointed out, that statement is fairly meaningless without further context, but I'm not the one who posted the context-free numbers. All I did is point out one conclusion from them. Another conclusion, of course, is the one probably intended by the original poster, e.g. that on average, black people commit more interracial murders than white people, but that conclusion likewise requires additional context before you could consider it to be actionable data.
Here's one piece of context for you -- "on average" has its own problems. As Dylan Roof illustrates, a single white guy easily supplied 4.5% of the carnage in that white-on-black number.
Should that make blacks feel safer or less secure?