I'm seeking clarification if a complementary landmark can be nested in the main landmark.

https://www.w3.org/TR/wai-aria-practices/examples/landmarks/complementary.html says "complementary landmarks should be top level landmarks (e.g. not contained within any other landmarks).", but SHOULD is not MUST.

The spec is kind of ambiguous: "A supporting section of the document, designed to be complementary to the main content at a similar level in the DOM hierarchy, but remains meaningful when separated from the main content."