I thought that as well, until I read Auguste Picard's 1937 balloon account, where he stated that, upon reaching his balloon's max elevation (don't remember how high he got) He stated that the "edge" looked like an "upturned plate". He left England, and landed somewhere on continental Europe. I would think that his use of "plate" implies he believed the "edge" to be round. Alas, I can't go beyond the 8oth south parallel to verfiry , per treaty.
Playing Devil's advocate, couldn't that curve we're seeing be the "edge" heading back towards the observer?, that would give the appearance of a curve, no?
Thx for posting. Wonder if they needed special permission to conduct launch?