"Black Lives Matter" is a movement that has traction because right now black people are being killed by cops. "All Lives Matter" is insensitive because it's ignoring the urgent problem that's affecting black people now. The trans-Atlantic slave trade ended centuries ago. There's no urgency to address something that has been over for that long.

You might see what they're doing as being pedantic, but I think you can acknowledge that the enslavement of Africans was a terrible crime against humanity without requiring a competition to see which atrocity gets the #1 spot. There's nothing about what the EU said that comes close to saying they don't want to take responsibility for what their ancestors did (although having said that, it's ridiculous to ask that someone ever take responsibility for something their ancestors did).

You can "whattabout" this all you want, but that doesn't mean that the declaration was a good one. It was a shitty one and shouldn't have been put forward at all.