I don't have this bag, but here's a review of it:
https://www.bayareafashionista.com/2016/08/23/proenza-schouler-hava-satchel-review/ And a video review:
It looks like the flap is indeed reinforced with stitching, plus I think the leather itself is pretty sturdy and not nearly as slouchy as in the old PS1 bags.
I agree, it's quite a nice and pretty timeless design.