H.265/h.265 make use of IPB frames. The P and B frames refer back to the I-frame so that each P/B frame has much less information than the I. So if you use only I-frames, that's the best that codec can offer, at a decent bit rate. there's much better descriptions on the web.
That doesn’t make sense, at the same bitrate encoding frames with no dependency on previous frames can’t possibly look better or be the best the codec can offer, or there would be no reason for other frame types to exist
They didn't say "same" bitrate but rather "decent" which I take to mean "sufficient" -- and obviously much larger than what you get with lossy(er?) params
