The slotted tailgate will let air into the bed - the interior of the bed under the tonneau will be a low pressure area - and that will cause the cover to try to balloon up and flap in a manner that it is not designed for. I'd expect it to act odd, at the least. I'm sure the rep knew what you were talking about, they probably get that question every day. What they are really saying is that the cover is not designed for that usage, so if you try it you're going to be on your own. I had a Truxedo on my last truck for five years, good cover, high quality, uses velcro along the sides but has a hard latch cover at the end where the tailgate comes up and normally seals the top edge. For a soft cover, it would probably do as well as any and better than most with an open tailgate. The Pace Edwards type of roll-up retractable hard covers would do a better job, I've driven thousands of miles with the cover half closed or the tailgate open with one on my current truck.
Brian
