So the KKK isn't a racist organization?
I don't see anything about the KKK on the billboard. If it wasn't in the thread I'd never know (and I'm not sure I saw a source). Of course the KKK is a racist org. That isn't the question.
If an organisation is racist and gives a weather report, does that make the weather report "racist"? If an organisation is religious, does that make what comes down "religious rain"?
Run this test: what if you find out tomorrow that you were mistaken and the KKK didn't buy the billboard and some heretofore unknown entity did --- does it somehow become "not racist" even thought it still says exactly the same thing?
What you have here is a Poison the Well fallacy; you've misused the term "context". Context is simply what the billboard
reads; it doesn't extend to "who paid for it", let alone "what their reasoning might have been when they bought it". The organisation may be racist; their motive behind it may even be racist. But they know by now they can't get away with overt racism to the general public, so they dance around it; instead of referring to blacks (or Jews or Catholics etc) they take the reverse approach and address whites. Another angle. Overt racism (probably) wouldn't be accepted by an advertising company anyway.
Strangely the top of your post already makes this point; stating a positive doesn't make for racism. For that you need to state a negative. What they have here may well be a lead-in to racism... but they've stopped short.
Test 2: you can make the point on this billboard without it necessarily being a setup for stating the negative about another race. In other words the negative conclusion is not inevitable.