"As in" has always been used to provide a concrete example of a word. I would say that it has always had a slightly sarcastic or incredulous tone to it - it could be used to describe, say, a piece of art "its made of carrier bags - as in what you take your shopping home in".
The use more recently as in the OP is just taking this one stage further - using an accepted linguistic trick to make a point. "Dead - as in no longer alive" can be used to emphasise the point to someone - along the lines of "when I say dead, I mean dead. Not injured, dead. No longer alive, never coming back". I have heard it used not ironically, but more in this form for emphasis, to make sure the person has understood, when they may be struggling to understand.