as far as I could see, there are only MISO, MOSI and GND pins requiered. A Digispark provides all of them. Therefore I would say: yes, it should be possible to use a Digispark ... but i haven't read the whole article.
Maybe it would be requiered to adjust one of the HEX files (owSilProg). In that case just use the offered support:
If for your microcontroller and/or pin assignment the appropriate owSilProg hex file is missing, then please don’t hesitate to contact me. In general, it’s easy to create a suitable hex file!
I guess it would also be requiered to use the Digispark USB driver instead of the one from the package. Maybe it would be a good idea to ask the author of the website to adjust/include the requiered files for the Digispark
