Everyone can now run certified real UNIX called openEuler made by Huawei of China, based on IBM red hat Linux. Certified UNIX distros are allowed to use the trademark UNIX® for their softwarez, so they are real Unices for what its worth. So openEuler is fully interoperable e.g. with Apple Mac OS X, IBM AIX, sgi IRIX et cetera which also are certified Unices.
Being UNIX implies being highly POSIX compliant.