That is correct.
I just performed a test using my Fast & Furious 4 UHD.
Test 1: CloneBD, video encoded 3840 x 2160 @ 25.24 Mb/s (ended up being 24.9 Mb/s) with copied DTS-X audio and no subtitles = 21.7 GiB and took approx 32 minutes.
Test 2: DVDFab, video cropped to 3840 x 1612 @ 25240 kbps (ended up being 25.0 Mb/s) with copied DTS-X audio and no subtitles = 21.7 GiB and took approx 21 minutes. (Frame Rate also changed from 16:9 to 2.40:1)
So, the only way I could think of to compare them is via "Bits Per Pixel".
Test 1: Resulted in 0.125
Test 2: Resulted in 0.169
Therefore, I say that the cropped version has slightly better video quality overall as there is no encoding bitrate wasted on the black bars.
Also, from a couple of other tests I ran, I deduce that the black bars take up less than 300mb overall.