The article references reporting by Nauka w Polske, which I believe is https://naukawpolsce.pl/aktualnosci/news%2C101748%2Codnalezi... which in turn references an article published by the library https://pressto.amu.edu.pl/index.php/b/article/view/38294/35... which has an English abstract:

The private book collection that belonged to Jakob and Wilhelm Grimm testifies to the sixty-year-long period of the fertile research (and artistic) output of the famous German researchers and founders of the German philology, and in itself is an important subject to study. This book collection, comprising rare prints and unique editions, is more than just a series of items of significant material of bibliophile value. It is particularly valuable primarily as a source of knowledge on the research methods used by the two researchers. In the footnotes and indexes to their publications of fairy tales or folk legends, both Jakob and Wilhelm were very meticulous in providing their written source material, i.e. the sources that they held in their book collection, the items of which had purposefully been acquired by the duo to preserve for subsequent editions of old relics European and German literature. Equally important for the research on the literary output and research activity of the Grimm Brothers is the analysis of hand-written underlining, notes and annotations that the texts were typographically enhanced by during the reading. The volumes from the portion of the working collection of the Brothers Grimm found in the Poznań University Library were long though to had been lost during the WWII, and now can be of significant value in their contribution to the development of modern research on the Brothers’ literary and scientific output. The very fact that they have been found allows us to believe that the book collections at the Library can hold other volumes that belonged to the private book collection of Jakob and Wilhelm Grimm still to be discovered.

So it's not about books written by the Brothers Grimm, but books which they used in their research.

so were they just sitting on the open stacks? Was someone just wandering down the aisle?

I can just imagine being a librarian and someone walks up to check the book out ...