The collision was clearly working, just some n% of the time you'd end up on the wrong side of the clean flat rectangle you were walking into.
I lost interest pretty soon after that.
Game development is full of domain knowledge like this because games need to make compromises ensure completing simulation updates at no lower than 30Hz.
The tutorial probably should have instructed you create box colliders for walls (giving a greater area for the physics engine to catch collisions) rather than a simple plane/quad which would lead to the exact issues you had, or at least explained why a box works better than a plane.
I guess you have to balance the necessary information with overload in your tutorial or at least have an aside or additional reading that really helps understand many of these internalized understandings.