If I had to guess, it would be something like Kaj’s example in the sibling comment—they were doing summations instead of loops, and they hadn’t seen the graphical arrangement of matrices that makes the multiplication obvious. (“Okay, wait, why are we using this index and that index?” → “You put matrix A here, matrix B there, you vector-multiply this row and this column and that creates this cell, and then you do that for all the cells.”) If you look at the Wikipedia page, imagine a class that only did the definition section and not the illustration section.
If I had to guess, it would be something like Kaj’s example in the sibling comment—they were doing summations instead of loops, and they hadn’t seen the graphical arrangement of matrices that makes the multiplication obvious. (“Okay, wait, why are we using this index and that index?” → “You put matrix A here, matrix B there, you vector-multiply this row and this column and that creates this cell, and then you do that for all the cells.”) If you look at the Wikipedia page, imagine a class that only did the definition section and not the illustration section.
Yes, it was a graphical explanation, I do remember that. (The one you describe sounds plausible, at least.)