Your basic understanding is correct, SLF3b. Must admit that I have missed most of the many no-doubt valuable contributions to the Spanair thread, due to the sheer volume of rubbish through which they are fighting for attention.
So I don't know precisely what argument you are referring to, but my impression is that a popular theory involves uncommanded (or at least undesired) reverser deployment on one side, possibly at max reverse thrust, with the other engine at take-off thrust. That would represent a much greater asymmetry than the 'normal' engine-failure-on-take-off case, and would probably be beyond the capability of full rudder to counteract at take-off speeds.
Tail-mounted engines are, of course, further apart than in the days of the turbojet Sud-Aviation Caravelle (1957), because even the 1980s turbofans of the MD80 series are fatter.