Okay, just checked with another input file *without* embedded fonts, and, yep, that seems to be it.
No fonts:
mobi6 EXTH Flags: 100001010000
mobi7 EXTH Flags: 1010000
Fonts:
mobi6 EXTH Flags: 1100001010000
mobi7 EXTH Flags: 1000001010000
So bit 12 seems to be the key here

.
Haven't checked if everything implodes if it's set and there's no fonts embedded, so, err, don't use the patch expecting everything will be rainbows and unicorns

...
(fontsignature is there in every case, so it doesn't seem to hurt or be of use for this).