In AD9826 datasheet, INL is +/-16 LSD, which is much great than +/-2 LSB of general precision 16 bits ADC.
What's the difference?
Your understanding is correct- the AD9826 was not designed for precision applications. As a result, it has a larger INL bow than a precision 16-bit ADC.
Why video(AD9826) application don't care the INL ?