I'd add NEC 250.118 as well -- that lists all the permissible equipment grounding conductors. Liquid-tight is listed in 250.118 as being an acceptable grounding conductor in lengths not to exceed 6 feet.
Yes, you understand correctly -- if you run an equipment grounding conductor inside the flex, you are essentially unlimited regarding the length of the flex. Beware that some local authorities might not agree with that statement, though, so I'd recommend checking with them first.