Hi, author of the article. I'm assuming urban traffic speeds, which is what I observe all the time myself, but you can look at the video of those kids, and count, and look at the seconds. 125 bikes in 45 seconds, between 0:02 and 0:47. Understanding it is another issue, but it's a fact. (This is one of those things that I do myself and would not claim that I exactly understand the details, I just do it.)

There have been more academic studies. e.g. https://nacto.org/wp-content/uploads/5_Zhou-Xu-Wang-and-Shen... estimates 2512 bicycles per hour per meter of road width, or 7536 bikes per hour on a 3-meter (10 feet) wide lane. That's only 4.2x car throughput, versus those kids who managed 5.5x.

You are right about the trailers, but at least where I ride, they are not common-case for carrying things, lots more cargo bikes instead, and those are "better" than trailers -- it's possible to ride two cargo bikes side-by-side even in a US protected lane (specifically on Garden Street in Cambridge, MA), though this of course assumes competent riders.