From the official NDK site:

The Android NDK... provides headers and libraries that allow you to build activities, handle user input, use hardware sensors, access application resources, and more, when programming in C or C++. If you write native code, your applications are still packaged into an .apk file and they still run inside of a virtual machine on the device. The fundamental Android application model does not change.

Yet, it is always described as a companion tool to the Android SDK and as a toolset that allows to "implement parts of your applications using native-code languages such as C and C++".

My understanding from this is that, unlike the Java based SDK, the NDK is not designed to implement certain parts of an Android application.

Is this correct?

If so, what parts doesn't the NDK allow implementing?