Indeed. It is in the manual and in the BMW pdf on oil usage.

it is NEVER a good idea to mix oil chemistries- oil is not just oil, but each brand (and even within brands) has other components- these should not be mixed. Period.

BMW is very careful only to have one "approved oil". Since they give free maintenance during the warranty period, they can do this (restrict the choice to their proprietary maintenance product)- they provide the oil during this maintenance period so this avoids the Moss-Magnussen restriction.

However, legally they would be required to give free oil to everyone to have for topping off...so they create an 'exception' and allow topping off with something else. Unfortunately, like so many things from this 'modern BMW', this is a recommendation driven by marketing and finance, and without much technical merit. Like lifetime tranny fills, and 15k oil intervals....

A