We apologize for any ambiguity or confusion that the update may have introduced - this was definitely not our intent. The update was aimed at making robot design and enforcement more straightforward. The short answer is that your case B is correct, not case A. For the long answer, allow us to explain our rationale a bit further.
Yes, a cylinder with infinite height makes sense for robot expansion theoretically and in CAD. However, after further review and feedback, we determined that it was not practically enforceable in the VEX Robotics Competition.
By making the maximum dimension a “point-to-point” width, rather than a true cylinder, it can be easily visually estimated using a standard yardstick or the diagonal of a standard VRC field tile (which is roughly 34"). The new Robot Expansion Sizing Tool that will be used by Event Partners this year is a “maximum width” tool, as seen here. Using this tool, there are some fringe cases (non-round shapes of constant width) that would not technically fit within a cylinder, but would still appear legal. See the image attached to this post for one example.
Ultimately, you are correct - a cylinder and a shape with “maximum horizontal dimension” are very similar. Thus, we feel that the majority of robot designs will not be affected by this change. However, the consistency with which robot expansion can be judged is now much more guaranteed. One of the primary goals of the Game Design Committee is always to ensure a consistent and fair experience for all teams involved in competition, and we believe that the revised expansion rule accomplishes that goal much more effectively than the original.