That is true, but for historical or historical-inspired games the playing field is usually a lot more level because the past is almost as much of a foreign country to present-day cultural "insiders" as it is to "outsiders".
If it came to, say, writing a supplement set during the reign of the Staufers, I would trust a British academic with even just moderate expertise in the topic over a random German. Or a random Italian.
Basically, if you want to write a historical supplement or an inspired supplement that "rings true" with authenticity, you need to put in the research, no matter what your citizenship or your ancestry is. Of course, that is often easier for natives of the countries in question due to one simple reason: Much of the quality research is going to be available only in the language of that country - and depending on the time period in question and the linguistic developments since, they may even be able to read primary sources.
Cultural familiarity, i.e. the ability to quickly unpack descriptions of past customs by reference to the presence, may also help, but it is IME secondary.
