That photo is certainly real. Small, cinched in waist was very fashionable back then, so of course her waist wasn't naturally that tiny. I doubt she was a size zero, she wasn't emaciated and even though she was slim, she had curves too.
This look of Brigitte is actually the one I like the most. I think she looked so much better as a brunette. As a blonde she looked like more of a product.
And about corsets, if worn properly, they can actually be quite comfortable. It feels like a firm hug, not painful or anything at all. Especially if it's made to measure.
Anyway, here's young Brigitte with her natural waist, wearing a bikini:
And here's more of Brigitte with cinched waist: