Strange that they list FreeDOS, DR-DOS and MS-DOS as separate OSes even though they are very similar. They are effectively ABI-compatible, unlike the different Unix flavors.
Almost, as everyone knows DR-DOS, MS-DOS and PC-DOS were like 99% compatible, the problem was when one was lucky enough to trip on that 1%, because they reverse engineered MS-DOS (even the IBM's PC-DOS license did not provide access to everything Microsoft was doing on MS-DOS).
Considering that DR-DOS is a linear descendant of CP/M with MS/PC-DOS ABI compatibility, it's certainly a separate OS. Same for FreeDOS. There's enough difference "under the hood" for each of them, however.
While I might stand corrected on that detail, OpenStep, OPENSTEP and NeXTSTEP had enough differences among themselves, with OpenStep also running on Windows, and many of the Sun's experiments with OpenStep own frameworks being on the genesis of Java and JavaEE.