I suspect the parent's statement might be accurate if you invested with another currency.

The problem (or uncertainty) is that both the US and Germany left the gold-backed systems during those years and the exchange rate between our currencies changed quite a bit during the 1914-1924 period (which is probably why your parent chose that year).

If you notice your image's description, it is denominated in 3 different currencies, depending on the years you are looking at.

You've always got to consider inflation over the long term, and in this case there is a well known hyperinflation period during the Weimar days. (I don't know about the GP statistic but this graph is clearly not meaningful)