Yes, that is correct. We can test your hypothesis by slightly modifying your example:
julia> @time begin
t = time()
Threads.@spawn (busywait(5); println(time() - t))
Threads.@threads for i in 1:Threads.nthreads()
busywait(1)
end
end
2.062341 seconds (43.90 k allocations: 2.938 MiB, 3.01% compilation time)
julia> 5.037096977233887 # printed after the 5s have passed