Okay, so, no, you don't. The difference is how Force operates. Force is when the GM chooses an outcome regardless of player input, choice, or mechanical result. It's a useful definition to isolate those things the GM is doing solely by fiat decision, and especially in disregard for player inputs. It's not inherently a bad thing -- I think 5e, for instance, effectively requires it to some degree. The adventure books are full of suggested Force.
So, in the example, the GM is choosing to deploy a specific example no matter what and in disregard to player input, choice, or mechanical resolutions. It's just Force. Again, not necessarily bad. Heck, I do this stuff all the time because it cuts down on prep. But, that doesn't make it not Force -- it's still Force.
Then we have Illusionism, which is a subset of Force where the Force is disguised or hidden from the players. Here the Force is hidden because they players cannot ever know that this would happen regardless of their choice. The cause is hidden from them. So, we have Illusionism.
If the GM made a wandering monster check and got the encounter and deployed it, this would be a part of the expected resolution of the game systems, and the GM isn't choosing this outcome by disregarding a mechanical resolution. Presumably, the players are aware that this is part of the game, so the result is fully expected to be possible.
Force and Illusionism are really talking about GM intent and how the structure of the game is working with regards to the back and forth of play. I don't think either are inherently bad, although a given player or GM might feel very strongly one way or the other on them. I don't mind a little bit of it, but I do mind pretty quickly, both as a player and a GM. I don't much care about it with regard to filler/flavor encounters, but I absolutely mind if any steps are taken with intent by the players that are then thwarted by the Force. For instance, if the encounter in the example happens if the players choose a fast or slow overland speed, then I'm in problem area -- these things should have weight in play and I'm discarding that by just throwing out an encounter regardless.