In my original post I did not explicitly say this, but the X7 M50i is NOT an M-car with an M engine (among other M specific components, like suspension and the differential for example, that distinguish M-cars from non-M cars). Only M-cars have the 1200 miles service called out, which includes the M2, M3, M4, M5, X3M, X4M, X5M, etc. For current M-cars, the 1200 mile service includes changing the engine oil and differential oil. Whether or not BMW will make a true X7M is yet to be seen, but that is a different conversation
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That said, treating any non-M BMW to the additional maintenance practices that are used on the M-cars is not a bad thing either (especially if you intend to keep the car for the long term), even if it is not part of the BMW covered maintenance plan. I did a 1200 mile oil service on my X7 40i (not the differential though), even though I had to pay for it out of my pocket - which is not that expensive (especially when you DIY like I do). Hell, my local dealer has oil change specials at least quarterly, like $49.99 for an oil change on any 6-cyl BMW ($79.99 for 8-cyl BMWs) which makes an oil change at the dealer a no-brainer (I just took advantage of this same special for my M3 not too long ago).
Your SA should have been able to tell you this about your M50i without the need to check the system, iDrive or otherwise. But the SA's lack of knowledge of stuff like this is not surprising unfortunately (and pretty common)...