Well the plugin is not a full release yet, in case that's not obvious. Anyway, YSlow provides some metrics that go beyond the load speed of a page and don't necessarily reflect it's real world performance. In the example provided the point is that the load speed cannot really be faster.
It's up to every site owner which optimizations they choose to make so it's actually a large assumption to assume what steps were taken by the owner. For now the plugin has some additional apache directives available in the ini/ directory of the plugin to set the remaining compression and header settings that would satisfy the remaining YSlow requirements provided that you use a properly configured self hosted CDN and configure YSlow appropriately.
There are so many factors to consider and too many options at the disposal of the users it's important that you actually work with the developer to get the results you need unless you actually read the FAQ and understand the plugin and what it actually does for your WordPress site.