Why does Potus say that his father was German?
Even when only his Grandfather was born in Germany and his father was born in the US?
BECAUSE IT IS TRUE!
In Germany, we had until January, 1 of 2000 the law that every child that was born outside of Germany to parents of German descent, is also German.
(Since Jan, 1, 2000 we added the rule that a child of non German parents could be also German, if it was born in Germany.)
Even if only one of his parents is German, their child is also German. This rule is called "Principle of lineage/ descent" ("Abstammungsprinzip")
https://www.bundesregierung.de/breg-de/suche/das-staatsangehoerigkeitsrecht-456726
Also important here:
"If someone is born in the USA and therefore is granted the status of a citizen of the USA (think Potus' father),
he is automatically granted also the German citizenship…."
[Wichtiger Hinweis: Wer durch Geburt in den USA die US-Staatsangehörigkeit und gleichzeitig durch Abstammung von einem deutschen Elternteil die deutsche Staatsangehörigkeit erworben hat, besitzt beide Staatsangehörigkeiten, ohne dass es eines Antrages auf Einbürgerung oder auf Beibehaltungsgenehmigung bedarf.]
https://www.germany.info/us-de/service/beibehaltung-der-deutschen-staatsangehoerigkeit/1216762
Summary:
Trump's grandfather was a German.
By the German "Rule of lineage/ descent", his child =Trump's father was also German.
Potus told the truth.
Again.