When gimp is built with USE='-pdf', gimp.desktop is installed with 'application/pdf' in MimeType. This causes programs to try to open PDFs with GIMP, which is unintentional and just won't work. Moreover, GIMP seems to take precedence over dedicated apps such as epdfview.
After further investigation I discovered that without poppler GIMP still supports importing (some) PDF files. So, I assume the pdf support is unconditional and thus the bug is invalid.