I'll counter that by saying, having worked with both, the internal functions and commands arent the same

Just a quick example, Unix manages its physical devices and hardware components in a different way than DOS- things such as mounting, the /dev directory, physical storage mediums treated as "real" directories.
Unix (1970's) was developed in Bell Labs(AT&T) using the C programming language. Linux was developed in 1991 (independent of IBM). I can only recall that MS DOS, OS/2, and Windows being developed as a result of IBM. IBM had nothing to do with Unix and Linux.