Acorn, Atari and Amiga were not DOS machines. There was a lot of variety of operating systems and computer hardware back in the 80s. It wasn’t until the mid 90s when the industry “standardised” (in a de facto sense of the term) on IBM-compatible machines. Before then most didn’t even run Intel CPUs let alone DOS.
I made a practical choice to focus on the IBM PC compatible GUIs because I am most familiar with them and I think they are some of the most poorly known due to an assumption that Windows was always the dominant option. And yes, I think it's fair to say that Windows 1/2 failed due to its lackluster comparison to other DOS options---DOS had significant market share that should have made Windows a top competitor, but it was not, as DOS users were choosing other options.
That said, you are absolutely correct that there were multiple competing microcomputer platforms at the time and most of them had GUIs on offer.
