If you’ve seen pictures of people from cultures who don’t wear clothing that supports breasts, you can see the difference in shape that constantly fighting against gravity makes.
There are a lot of factors that can contribute to sagging, but bralessness doesn’t seem to be one. There are studies indicating that bras likely weaken breast tissue over time, making sagging more likely in people who wear them regularly. Of course, even those studies are tricky to generalize to the broader population because of how many factors there are to control for (breast size, pregnancies, tobacco use, genes, etc etc etc). What we do know is that the difference in breast shape from one whole ethnic group to another is largely attributable to genetics, and as for the photo: those women simply have very different bodies.
Interestingly, photos of African women have been used countless times throughout history to dishonestly market bras to fashionable Westerners. The image of African women who simply have a different body type from their own has frightened millions of white ladies into bras.