First of all, they aren't suing Greece or all of Europe.
Second of all, the apology is ... it goes like this. When you build an empire on the backs of people, 100 years later, those people on whose backs you used are living in abject poverty as a direct result of their enslavement, displacement, policies which stood to keep the elite on top and others on the bottom. Meanwhile the empires which benefitted from the wealth are now internationally known for their good wealth, opportunities, and "developed" nations. Their people are living quite well and in and international economy fair better because of their connections, skills, etc. Meanwhile those in poverty are expected to pick themselves up by the boot straps in a world where the odds are not even, but in fact stacked against them. Not only that the legacy of slavery and displacement is still felt immediately in the families and descendants.
Now you, mighty men of incredible good luck and opportunities are reluctant to help, and eager to wash your hands of the abuses which your fathers household inflicted upon your poor neighbor. And it is so easy to 'cop' out and say, well every had slaves right?
Thats like a thief being held accountable saying, 'but hey other people steal, shouldn't you be going after them instead?'
Thats not a defense lol.
As for the apology, one highly underestimates the power of such a thing if you feel you are only apologising to dead people, and feel you have no place in the legacy of that. A lack of compassion and responsibility is what that is.