Flash memory slower than magnetic disk
The quote in the question is wrong. Thumbdrives use the same flash-chips as SSDs, but there are architectural differences.

In a high-performing SSD, you have maybe 8-16 flash-chips in parallel and a controller with a very wide interface that can access all chips simultaneously. This masks latency of the flash and of course makes the r/w rate much much higher than a single chip (and much higher than a magnetic disk).

See this image of a modern SSD PCB. There are as many chips on the bottom side as well.

In a thumb drive you might have a single flash only. 

Also, apart from the hardware, due to the block-based nature of the flash and the fact that blocks wear out, the controller firmware is crucial in making the balance between raw performance and longevity. This is the most important difference between SSDs in performance. If it hadn't been for this, there would not have been such a huge market of SSDs and there wouldn't have been reports all the time that new designs fail and give abysmal performance after a while.. 

In a cheap thumb drive with much fewer chips in parallel the firmware might not be that thorough and can't hide for example erasure latency as well.

You also have the difference of the interface itself, USB vs. SATA, although in my experience this is not the limiting factor for slow USB thumb drive performance.