He's not though. He was probably given some version of that statement as part of a briefing, but more importantly his businesses and campaign finance were incredibly entangled with Russian money and oligarchs at pretty much every level[1,2], not to mention his bizarre interactions with Putin.
This was his version of simply yelling the same insult back at people he didn't like: and he really didn't like Merkel (or any European leader) because unlike the despots and dictators they wouldn't just suck up to him (notably Shinzo Abe figured him out immediately and went all in on the obvious play for favor[3])
My explanation is that it doesn’t actually take a lot of deep thought to understand this dynamic. The guy who runs your local hardware store could’ve figured this one out, given the information.
The problem is that the over “educated” and credentialed class of political elites weren’t ever taught elementary common sense in their textbooks, and have never operated within institutions that have held them accountable for it.
Ergo the outsized elite hatred for Trump among the managerial elite caste, who shone a spotlight on the astonishing triviality of their ineptitude.