No analogy is perfect. They are intended to help illustrate a concept to gain insight. Ideally when using an analogy its limits should also be described. Luckily the very first link after the analogy is explained does just that: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir3.....
My understanding was that if you write a regular function (`def` rather than `async def`) then FastAPI (or really Starlette which it uses under the hood) executes the function in a thread pool so that no blocking of the main event loop should occur.