It's not total bullshit. If a white guy and a black guy spit into two different tubes and you run two DNA tests, it will tell you with 100% accuracy which tube belonged to who. So at the continent level at least, it's not bullshit at all and is in fact highly accurate, but when you start getting down into individual countries it gets murkier.
Do you have a link about identical twins getting different results? I think it's expected that you get something like 1 read error every one or two tests, so even a small error like that could throw off the results somewhat. But they shouldn't have gotten radically different results.
My ancestors DID own slaves. Still don't give a shit.
The shills are the people who want you to shut up and deny reality over fear of "looking bad".