It's not about velocity it's about rate of cars through a point per hour. If you have a mergepoint with 10 lanes. Each lane can support say 20 cars per minute. If you have 200 cars approaching the merge-point per minute the cars can travel effectively at the speed of their choice.

If you close one lane, reducing the capacity of the mergepoint to 180 cars per minute while 200 cars are approaching a queue will build. The speed with which the cars mass the mergepoint is not relevant because the rate of cars per lane per minute will stay basically at 20 cars/lane/min.

As to your point about all lanes slowing down - cars will always redistribute as you can imagine. People tend to merge left as there's an additional traffic stream merging on their right. The writer made points about the capacity of the mergepoint vs. the cars approaching- not individual lanes and speeds.