What if you don't know you don't have that control? The techniques described largely revolve around the players having the illusion of control. If they choose to go to City A, where they discover that the villain is drinking coffee in the cafe across the street. They choose to go to town B, the villain is drinking ale in the pub down the block. Or with a different example, the extradimensional invaders set up their beachhead wherever the players happen to be.
If you do not know the world is adjusted so the action and plot always happens around the PCs, is your enjoyment of that action effected? The argument they're making is that what you do not know does not lessen your enjoyment of the game.
I don't think that the trick of "illusionism" as described is really good enough to fool people except under the most limited of situations. So far, the only examples of illusionism described involve the players making minor decisions: which city to go to, which road to take, whether they go to meet the necromancer or wait for him to come to them, etc. In all of these situations, the DM already has encounters and events worked out ahead of time that he can adapt slightly to place in multiple different situations.
However, it is not possible to justify the same encounter in every situation. For example, suppose the DM has planned out an encounter where the PCs leave the city and fight a gang of bandits on horseback. This encounter can work no matter which road out of the city the PCs choose to take. But, in a surprise twist, the PCs decide to travel to a completely unexpected destination, by ship. Suddenly, the DM is in a bind, since his encounter makes no sense if the PCs are at sea. He is forced to do one of two things: prevent the players from leaving by ship (railroad), or toss his planned encounter out the window. The PCs could alternatively teleport to their destination, stick around in town and pursue local politics/crime adventure, or so on as well and cause the DM the same problem.
I guess that I will admit that illusionism is not really bad. But it is not some kind of fool proof strategy to avoiding railroading. Since it is nothing more than a DM adapting his planned adventure to different circumstances, it isn't really a useful tool for dealing with PCs who behave in a completely unexpected manner. At the end of the day, the DM is still forced to decide between railroading the players so he can still use his planned encounters, or tossing his notes out the window and letting the PCs do what they want.
Also, while illusionism may be impossible to notice when done well, it is pretty obvious when illusionism
isn't being used. For example, the players are given the choice between hunting down the evil necromancer, and hunting the dragon who possess the artifact that destroys undead. In this situation, the nature of the two adventures is so different that no encounter can be shared between them. So, the players are making a meaningful choice, not an illusionary one. I think a player can notice the difference.