I'm not sure what to suggest. All the digispark I have came with bootloader installed. I have used both ArduinoISP in Nano and USBasp programmers to burn "bootloader" into bare ATtiny85 dip chips, using @DrAzzy 's core. But I never needed to burn the digispark bootloader. The instructions you found sound ok.
Perhaps I could try your sketch on one of my digispark to see if I get the same result. That will need to wait until the weekend, if I get any time!