I don't play online cash games, so this article is really only theoretical for me. It all makes sense, and is logical to follow.
For my purposes, "multi tabling" refers to playing more than one MTT at a time. I have found some of my limits in this task. Straight up "how many tables can I play?": I think my maximum is three before success starts to tail off. If one of the tables goes to heads up, it will suck more of my time and attention. If there is inequity in the stakes, attention can also be diverted to one table over others. I regularly play four tables of MTTs at once, but my play is not optimal. As soon as two of three or four tables go heads up, I realize that I'm maxed out, or overwhelmed.
My biggest issue is playing multiple tables on multiple sites. As a CardsChat freeroll addict, I can regularly find myself playing four tourneys on three or four sites simultaneously. This is a disaster, but the solution is a better computer, not a change in game approach.