This, but they missed the opportunity to call it a single I-Frame with only P-frames in each direction, if they actually did some effort in terms of compression

This, but they missed the opportunity to call it a single I-Frame with only P-frames in each direction, if they actually did some effort in terms of compression efficiency. Another thing is that any 4k capable codec is also probably magnitudes more space efficient than baseline jpeg for stills, with or without added temporal information.