I believe in English (at least in the US) there is a distinction between a "Server" and a "Servant", the former being the "tjener" that wait on you in a restaurant while the second is the "tjener" that serve you (and your every need etc.). I expect that in general when "serving God" it is in the subservient meaning.... although I suppose God could go out for the occasional meal in a fine dining restaurant if He wanted to :-)
I know it supposed to be servant, but absurdity is not far from Duolingo :)
I wrote that and it was accepted and no, it would still be "Engle er Guds tjenere." It is only the English that mangle the language like that, ha ha.
I mean, you could write "Engle er tjenere af Gud." but nobody would say a thing like that. In English you say it all the time ... "capital of Sweden", "flag of Denmark" and "owner of the dog", but you would still translate it to "Sveriges hovedstad", "Danmarks flag" og "hundens ejer" and not "hovedstaden af Sverige" etc.