Bolding other sentences in the wikipedia you quoted makes the exact point you're looking for:

Nearly all the wartime documentation on Generalplan Ost was deliberately destroyed shortly before Germany's defeat in May 1945.[3][4] Thus, no copies of the plan were found after the war among the documents in German archives. Apart from Ehlich's testimony, there are several documents which refer to this plan or are supplements to it. Although no copies of the actual document have survived, most of the plan's essential elements have been reconstructed from related memos, abstracts and other ancillary documents.