In principle, it is a non-orthogonal access, since the users transmit over the same resources; however, such a type of access falls under space-division multiple access (SDMA). So no, it is not 3GPP MUST and nor conventional NOMA in a direct sense. Moreoever, in order to benefit from NOMA, you need power-domain multi-user detection, which can be based on a simple interfernce cancellation or through a more-complex joint maximum-likeihood search, which are both not enabled anyway when MUMIMO modes are used.
In the current release of the simulator, only downlink NOMA is supported, and it is enabled by setting multiuserMode.Downlink to MUST. In this case, joint maximum-likelihood detection over the composite constellation of the near- and far-UEs is performed at the near-UE, while the far-UE detects its signal directly ignoring the interference by near-UE.