Yes, and together with BoB trailer weighs 18 lbs. I owned and weighed them. Weigh one yourself if you doubt the information.

I wonder why you would question this data. A pair of Ortlieb BRC panniers hold 2441 c.i. and weigh 4.2 lbs. Is it so hard to believe the YakSak, made of basically the same material as Ortlieb BRC, and rated at larger 5,600 c.i. volume (another exaggeration by BoB, closer to 4,000 c.i. with a single folding closure) weighs ~4.5 lbs?

Most would disagree with your view that the Yaksak is unnecessary. In USA Bob Yak trailers are sold with sack included - so there's no reason to make your own sack. Most folks like their gear in a tough waterproof bag, which is a big selling point of the BoB Yak (and Ortlieb BRC).

I have made extensive comparisons of weights/capacities of Bob/Burley/Ortlieb and posted in this forum on several occasions. Google if you care to learn more.