As I understand Trump is more isolationist and less neo-con interventionist than Biden/Harris is. So why does the article state 'increased geo-political challanges' after a Trump win?
Because Europe cannot put trust in US help for Ukraine. Thus, Germany has to do more. (and they can only do more if they spend money and the [now ex-]finance minister does not want to)
Also, if the US raises its tariffs wrt to Europe, then the German economy is in even more trouble.
Because Russia will conquer eastern Europe and China Taiwan.