I misunderstood the question. I think the question is why both "господину" and "Цукербергу" are declined, given that "Цукербергу" has non-slavic origin. I think

I misunderstood the question. I think the question is why both "господину" and "Цукербергу" are declined, given that "Цукербергу" has non-slavic origin. I think typically the rule is that only names of Slavic origin are declined along with generic name, such as "в город Минск", "в город Киев", "в город Москву". But the foreign names it is not, e.g. "в город Сан Франциско", "в город Сан Сиэтл". I think it definitely true for geographical names, and probably for last names as well. However, even for geographical names there many names that considered long assimilated, e.g. "в городе Санкт-Петербурге". I think the same true for a last name such as Цукербергу. There are plenty of people with Germanic last names, especially ending with -berg, -man, -baum, etc, so maybe it is considered assimilated enough that it follows rules for Slavic based names.