That is (partially) true: in most cases you don't need to touch the ARM7 core at all (as for the official SDK), because the library provides a default arm7 core. In fact, using the ARM7 core is even discouraged now.
By the way, the compiler itself takes care to call ndstool.exe (the tool that packages arm7 and arm9 binaries), so it spits out a working nds ROM in a single step.
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