Generally they would neck it down at the top.

Since the ID is fixed at 27.2 to take a post and a wall of something to either side of 0.7mm, so it might be a 30.0mm tube, necked down to something roughly 28.6mm. The math doesn't always work because necking tube down, might produce a slightly thicker wall. (this is controlable with mandrels so it's a design consideration, rather something that just happens).

Necking a larger tube down to accept a standard post is common with Ti frames, and suppose is possible in steel, but 30.0 doesn't conform to any of the classic standards. Also, it's a decent amount of work and added cost for such a small change in OD. But that said, I'm not in touch with some of the creative stuff happening out there.