There's actually an English version (i.e. Der Spiegel International translation) of that article, but it has a somewhat different title and doesn't mention Ukraine in the summary, so it's a bit hard to find/relate. It refers to the same print edition 18/2022, that's how I know it's the same piece referenced.
The article is mostly paraphrasing with only the briefest of quotes from actual/original documents. But there there are two primary sources quoted there in re Ukraine: one is Kohl's conversation with Mitterrand "in early 1991" and then 2nd a November memo summarizing a discussion between Kohl and Yeltsin, a couple of weeks before Ukraine's independence referendum.
The new [declassified] volume with papers from 1991 includes memos, minutes and letters containing previously unknown details about NATO’s eastward expansion, the collapse of the Soviet Union and the independence of Ukraine. [...]
In 1991, the Soviet Union was still in existence, though many of the nationalities that formed the union had begun standing up to Moscow. Kohl, though, felt that a dissolution of the Soviet Union would be a "catastrophe" and anyone pushing for such a result was an "ass." In consequence, he repeatedly sought to drum up momentum in the West against independence for Ukraine and the Baltic states.
[...] Kohl found himself faced with the three Baltic republics pushing for independence and seeking to leave the Soviet Union, Kohl felt they were on the "wrong path," as he told French President François Mitterrand during a meeting in Paris in early 1991. Kohl, of course, had rapidly moved ahead with Germany’s reunification. But he felt that Latvia, Estonia and Lithuania should be more patient about their freedom – and should wait around another 10 years, the chancellor seemed to think at the time. And even then, Kohl felt the three countries should be neutral ("Finnish status"), and not become members of NATO or the European Community (EC).
He felt Ukraine should also remain in the Soviet Union, at least initially, so as not endanger its continued existence. Once it became clear that the Soviet Union was facing dissolution, the Germans were in favor of Kyiv joining a confederation with Russia and other former Soviet republics. In November 1991, Kohl offered Russian President Boris Yeltsin to "exert influence on the Ukrainian leadership" to join such a union, according to a memo from a discussion held between Kohl and Yeltsin during a trip by the Russian president to the German capital of Bonn. German diplomats felt that Kyiv was demonstrating a "tendency toward authoritarian-nationalist excesses."
When over 90 percent of Ukrainian voters cast their ballots in favor of independence in a referendum held two weeks later, though, both Kohl and Genscher changed course. Germany was the first EC member state to recognized Ukraine’s independence.
Three's a bit more about NATO expansion in there, but since my Q wasn't about that, I won't get to the remained of that article.