Arduino_Boards VS micronucleus

You can actually bit-bang low speed USB on Arduino [0]. There is even a bootloader [1] for ATtiny. It is obviously not compatible with Arduino, but those tiny boards look too cute to ignore [2]. Also, you can use USB port both for downloading firmware and your application.

[0] https://github.com/obdev/v-usb

[1] https://github.com/micronucleus/micronucleus

[2] https://cpldcpu.wordpress.com/2014/04/25/the-nanite-85/

The Digispark[1] and its clones use the ATTINY85 and offer flashing over USB via the Micronucleus bootloader[2] using V-USB[3]. It comes at the cost of a significant chunk of flash storage, but the convenience is excellent. Plus, V-USB support means you can emulate all sorts of USB devices. I've used it to build a DIY NES controller adapter, but it's commonly used as a cheap Rubber Ducky[4]. For projects where USB interfacing makes sense, it can be a good fit.

1: http://digistump.com/products/1

2: https://github.com/micronucleus/micronucleus

3: https://www.obdev.at/products/vusb/index.html

4: https://shop.hak5.org/products/usb-rubber-ducky-deluxe

So it looks like they're using the micronucleus bootloader for the linked example. That bootloader not only does the normal bootloader job, but also acts as a bitbanging USB device in order to communicate with the computer over USB. The USB bitbanging is a huge endeavor to undertake on top of writing a bootloader.