Not a "clone" in the sense of 100% compatible... but a "clone" in the sense of borrowing a significant degree of ideas and APIs/syntax from it, yet also deviating at various points (whether for good reasons or for idiosyncratic ones)
It is a clone more in the sense that the Japanese mainframe operating systems Fujitsu MSP and Hitachi VOS3 are/were clones of IBM MVS: they started out as a direct copy of MVS–due to IBM having released earlier versions into the public domain, and Fujitsu/Hitachi illegally stealing the IP of later ones–but over time diverged in incompatible directions
All real mode programs that are compiled with Watcom C/C++ should work. The most recent versions of Watcom's protected mode runtime don't currently work, because they use some undocumented MS-DOS syscalls that are not implemented in ST-DOS. I intend to create a compatibility TSR that will solve most issues with those MS-DOS programs.
The documentation claims things (which I have not verified personally) such as forking, signals, and pipes. Instead of a SUBST command there is a MOUNT command. And there's a convoluted piece of hooplah that stands in place of what MS-DOS could do with the SHELL= line in CONFIG.SYS .