it would be incorrect to say that it ever was their main product.

They made several full versions of it... It was not simply a one off product.

While Xenix 2.0 was still based on Version 7 Unix,[30] version 3.0 was upgraded to a Unix System III code base,[12]: 9 [31][32] a 1984 Intel manual for Xenix 286 noted that the Xenix kernel had about 10,000 lines at this time.[10]: 1–7  It was followed by a System V R2 codebase in Xenix 5.0 (a.k.a. Xenix System V).[33]

Also,

Microsoft's Chris Larson described MS-DOS 2.0's Xenix compatibility as "the second most important feature".[38] His company advertised DOS and Xenix together, describing MS-DOS 2.0 (its "single-user OS") as sharing features and system calls with Xenix ("the multi-user, multi-tasking, Unix-derived operating system"), and promising easy porting between them.[39]

So they were simultaneously created AND interoperable (from a program development perspective). This was a full fledged item.

Edit: to elaborate a little better. If they were simultaneously developed... and interoperable. And one item is Unix-based outright. Then it's safe to say that the other item (MS-DOS) in this case is also pretty steeped in Unix roots.