OK, but Althochdeutsch didn't appear out of nowhere, it's just the oldest written form of German. Before that it was called (West) Germanic, Latin's sister, and as Germanic developed, it took on lots of other Latin words from the neighbours, and the result was - AHD!
Some other words which came from Latin (Roman legionnaires, etc) to Germanic before it became AHD are Strasse, Tisch, Zoll, Keller, Kürbis ... (can you tell I have a list?) ...Eimer, Esel, Kessel ...etc.
Why make a distinction between words in the language before anyone wrote it, and words in the language since then? And then call the latter "foreign"?
I've heard the argument that Fremdwörter don't follow the rules of German pronunciation. So, for example, German 2-syllable words are pronounced with the stress on the first syllable, but Fremdwörter such as "nasal" have the stress on the second syllable. However, it seems a bit silly to me to say that 2-syllable German words always have the stress on the first syllable if half the words spoken by Germans don't - but to make the rule still fit you just say the others are foreign! (Natur, Kultur, Magie, Beruf, Ballett, Pension, etc.)