I guess this was one of those "inventions" which are pretty obvious in hindsight, and I bet more than one party had already though of building a USB drive, it was just a question of the USB interface becoming widespread enough and of flash memory becoming affordable and reliable enough. Actually, flash memory cards with proprietary interfaces were available for some time before the year 2000 given in the article (Compact Flash since 1994, Sony's Memory Stick since 1998), so it was just the question of using USB instead of these interfaces.