I'm saying it's casual to them because they are *so* used to touch each other's bodies. We have seen them put their hands down each other's pants in front of cameras, that's how far they are willing to go, so kissing an arm is as casual to them as shaking hands to most of us.
And like
cool said, kissing arms is indeed casual in some cultures, and the Russian culture is one of them.
As for the "scope" of their relationship. What i mean by that is that there is no reason to be amazed by what is shown in that video. It's a pretty common display of affection and caring between friends, it's cute, but it's in no way extraordinary. Millions of people would have done the same thing in a similar situation.
In their current state, Tatu is no longer breaking any barriers or pushing any limits in terms of what kind of relationships are socially acceptable in our societies. They are now two close female friends who love each other. Totally banal and common.
But it *is* cute! I agree there.
They are both so pretty, what's not to like.