I guarantee you that no statistically noticeable number of new Mario players ever managed to play for 5 minutes without figuring out that the mushroom made Mario more powerful.
I understand that. However, the parent post (among other online sources) says that they made it impossible to avoid the mushroom, which is not true.
I think this is an important distinction, since they clearly could have put the player in a situation where the mushroom is both unavoidable and necessary to advance. They could even had done something as heavy-handed as tutorial shown in the article. Instead, they chose to put the player in a situation that was indistinguishable from regular gameplay, but biased to educate the player. This, I feel, is a more subtle approach to game design than forcing the player to a prescribed route.
You’re taking it too literally. The point was that they made it so that players would necessarily quickly learn that the mushroom wasn’t an enemy, even though other things that looked nearly like mushrooms were enemies (as was learned the first two or three plays by dying to them). If it was easy to avoid the mushroom, some players would spend many plays through carefully avoiding it. “Force” is relative.