No, this is a different feature. Command-E transfers the selection in the "Find" text field (and Command-Shift-E transfers the selection into the "Replace" field). That's different from a "Find Within Selection" function.
My advice to developers running into problems like this is to submit a bug report, and don't waste a lot of time venting about it. Some of this may be oversights, or features deferred due to lack of time, or features available in some other form (but possibly not in an obvious place). The bug report makes sure the Xcode team knows what's missing, and — most importantly — votes on what missing features are most important to the developer community.
Bug report early and often.