Sometimes you can organize your work such that while yes, technically you're just doing one activity, you're making progress on multiple todo entries simultaneously.
I think that's an important facet of effective "multi-tasking"; it has more to do with how you organize your work than being good at context switching. Sortof akin to enabling SIMD use by organizing the data such that it's applicable.
A practical example is I have this long-term excavation/grading project on my property that needs eventual doing. The dirt here is mostly sand, and I often mix stucco/concrete for projects. By deliberately sourcing the dirt from areas I need to excavate anyways, I'm making progress on the grading project when I'm making pavers or whatever needing sand.