It absolutely is, and has been for a long time. I find it very hard to keep track of the various standards, but things got a lot simpler with v3 of the Single Unix Specification, which is the same as POSIX 2001[0].
The problem is that SUS and POSIX only define a subset of what's wanted by most modern software and a subset of the tools people need to interacting with the system. It's a double-edged sword, though: if we always stuck to what was common to all Unix-like systems, we wouldn't have lots of important OS features.
While correct, POSIX is supposed to mirror UNIX, but only does so partially and like ANSI C, leaves room for implementation specific behaviours and extensions.
So in the end, the situation isn't much better, if the goal is to reach as much UNIX like systems as possible.
