If you need thinapped application to see the dll that is copied on the system, verify the isolation mode for the folder in the project where the dll resides is set to merged or writecopy.
As far as ExternalDlls is concerned, its an over-ride we use to load dlls using the window's native loader.
In general, (there are exceptions to this rule, such as side by side dlls etc.)
If a dll is part of the package, it will be loaded by the thinapp runtime, all dlls residing in the system will be loaded externally.
-Neeraj