I believe they are stretching the envelope a bit. ~10-20MHz is probably the reliable top end for the RPi GPIOs, and even early SCSI standards well exceed that at 5MB/sec. That's taking the rather naive assumption that 20MHz is 20 mega bits (not bytes)/sec. Various encoding ideas could make that better, or worse.

This is a cool dancing bear and all, but I would stick with the microcontroller implementations for real needs. This is cool to experiment with.

SCSI2SDV6 is supposedly capable of 10MB/sec. There's also a guy that designed something with a very capable teensy and a real NCR 5380 SCSI controller. https://www.pjrc.com/tiny-scsi-emulator/

Edit: The Teensy is pretty impressive in this general area. Teensy 4.0 is a features an ARM Cortex-M7 processor at 600Mhz, 1024K RAM, with lots of community support, and it costs $20. You can imagine it blurs the lines a bit on "microcontroller". Bitbanging GPIOs, in bare metal mode, on it will likely work a lot better than an Rpi/Linux. https://www.pjrc.com/store/teensy40.html