I'm not sure that I recognise tango as strict tempo dance music - you overstate the regularity of the pulse at times, surely?
Occasionally, the tempo will change at the ending of the song, and even rarer, the intro might have a different tempo from the rest. Also, in some Pugliese's, there are tempo changes (which is why occasionally you'll here someone claim they are undanceable).
Having recently watched the video in the OP, IMO they are not stepping off beat, but are simply using the "off beat" a fair amount.
To try to explain, if you count the "quick beat" in a tango song (repeating after 4 quick beats), you mostly would be stepping on the 1 and 3 beats (or the slow beat). To do a quick-quick-slow rhythm, you could use the 1, 2 & 3 beats (and not the 4).
However, one could use the 2 and 4 beats, which could be called the off beat. That's what I think Horacio was referring to.
Stepping someplace, but not on any of the 1, 2, 3, or 4, would be called stepping off beat, but again, I don't think that's what Horacio was trying to teach.
For the musicians here: I do realize that one could step on a even split of those beat (like the really quick beat found in some tangos), without being off beat, but I'm trying not to get too technical.