MS-DOS was also not the only DOS on the market. DR-DOS was very popular at the time of the Atari ST. As far as I know DR-DOS also followed the published specification and Digital Research (DR) was Microsoft's biggest competition. Atari also used Digital Reasearch's GEM as their "desktop" GUI. GEM was a direct competitor to Microsoft's new Macintosh-killer product called "Windows". CorelDraw and WordPerfect used GEM, MS Word and MS Excel used Windows.

Draw your conclusions.