That is an artist rendition of the Mavic flying, certainly not an actual picture of it in action. I'm not sure if I'd say such a thing is proof that it's okay. However, DJI rep's in the forums state that it can be left on. However, this is also from a company that allowed the batteries to turn off mid-flight if they got a little cold.
I'd place heat up there with water when it comes to things that kill electronics. I weight this against the possibility that the cover will actually make a difference if the drone crashes and that any sun glare will show up on the cover and basically ruin the shot. Do I think it would hurt to leave the cover on for some runs? No.. especially in cooler weather. Keep in mind, the Mavic came out the end of October and most people would be in cooler weather at that time. I don't know that it's had extensive testing in hot temps yet. As mentioned above, DJI has screwed up their designs in the past. I also think that DJI rushed out the Mavic. After just a few minutes with it I can find some design issues. I think DJI rushed it out to try to kill the Karma.