I'd like to believe that (though Business Server doesn't even have the excuse of being in German language) but I can't find anything about that term on the linked Wikipedia page.
My guess would be that it originally did mean "Betriebssystem", and the "Business Server" was coined a lot later for the non-German-speaking part of the world.
And have a look at [1], if you compare "PBS" (Platte Betriebssystem) to "BS 1000", both running on a 4004-45, it would be really really strange if this "BS" meant "Business Server" in 1968.
[1] https://de.wikipedia.org/wiki/Liste_der_Siemens-Computer