You misread. The claim isn’t that Ukraine was the initial aggressor against Russia in the war. You might need to review again and recognise that different events and claims are being referred to. The aggression here is about the post Euromaiden actions that led to the Donbas counter protests later crushed by the state. Aggressor here refers to the state, the belligerent in that sentence is the opposing side in the civil war. Russian speaking Ukrainians.

So another way you to read this is:

1- Donbas has been lost for a long time

2- long before the Russia Ukraine hot war, there was a longer pre invasion civil war

3 - during that civil war the state was the first aggressor towards its own population who have been threatening to secede for over ten year without constitutional reform.