8bit are correct. I have just checked this on various test sites. An error has crept into the specifications.
Good eye, I would never have noticed such a thing ;)

Sources:
https://www.gamestar.de/artikel...
http://www.tftcentral.co.uk/reviews...

The fact that Dell's technical specifications state 16.7 million colours speaks very much in favour of 24bit. Are you sure it's the same model in the promotional video? 8bit would also be a bit weak for the price. I would always rely on the text rather than the picture/video.
Do you have a link to the video?