This means although it is and I-frame, the decoder might still use/need previously sent frames for decoding. In contrast if key_frame=1 this would be an IDR-Frame (Instantaneous Decoder Refresh).
Citing from Ian E. G. Richardson's H.264 and MPEG-4 Video Compression:
An encoder sends an IDR (Instantaneous Decoder Refresh) coded picture (made up of I- or SI-slices) to clear the contents of the reference picture buffer. On receiving an IDR coded picture, the decoder marks all pictures in the reference buffer as ‘unused for reference’. All subsequent transmitted slices can be decoded without reference to any frame decoded prior to the IDR picture. The first picture in a coded video sequence is always an IDR picture.
This may have impact on random access to a video. Simply said, IDR frames make random access more robust.
