Each additional lane has less and less impact because of lane switching. Ultimately, you can still only enter or exit on the left or right, regardless of if you have 100 lanes. And having people move across 100 lanes to exit is much slower than moving across one or two.
I don't doubt it. It is quite a while ago so I don't fully recall the talk that my professor gave, but I don't believe he intended to mean adding lanes was useless, just that they didn't help with congestion of the particular roadway
If adding lanes doesn't help you didn't add enough. People live in a city to do things and you need to enable that by preventing congestion.
i don't know how to afford the 50 more lanes that most cities need though. I suggest better transit.
Removing parts of the city to add more lanes will also help decreasing demand - it's a win-win!