Not only Germans does it, but everybody who writes in German language. (German is official language of Austria, parts of Switzerland, Liechtenstein, parts of Italy and many other regions too, where mainly people live who are not Germans, and they all write nouns with uppercase first letter when they write texts in their native language.)
2000 years old ...
My best guess is, the Der can be found below the Ipswich peninsula, the same way as the trailing an of Ocean in the upper part of the excerpt.
As you can check on DWDS, in the Etymologie section:
no other gender than masculine is mentioned
Latin oceanus and Greek Ὠκεανός were also masculine
So I doubt any intermittent deviations. Brockhaus 1894 also has it ...