I think you may have misunderstood the comment you are commenting on. I believe they were saying that members of the socially dominate group who perform crimes are seen as outliers while members of minority groups are seen as reflecting the values of the group.

it also have to be politically motivated too, but conducted by someone alienated by the populace (i.e war doesn't count)

and white lunatic doesn't count as a terrorist, but dylan roof or eliot rogers does.

In-group is a sociology term. He is saying it is bigger/different from whether or not the individual is white, but whether they are a member of the in-group.

I don't think the parent means a group of people committing a crime - they mean 'in group' as in US citizen/not the enemy.

That still doesn't make sense. Domestic terrorism is a thing and has been for decades.