Yeah, word definitions. To me flat is flat, not warped. Smooth is smooth, no ruffles.
The leaf spring needs to be flat and smooth to get the best possible "seal" with a metal-to-metal interface between the leaf spring and end cap. A leaf spring can be smooth, but warped and cause a leak. A leaf spring could be flat and ruffled (not smooth), and cause a leak. If it's both not flat and not smooth, then possibly even a bigger leak.
If an engineer specified how the leaf spring should be made, the "sealing" area would have both a flatness spec and a smoothness spec. Ideally, not warped (flatness) and no ruffles (smoothness).