sync contextから Task.init するときにどうなるのかが気になります

This design decision was made specifically to make the behavior of Task.init consistent across nonisolated synchronous and async functions. We can't change the behavior of unstructured task isolation in nonisolated synchronous functions,

because that would require changing the ABI of every unspecified synchronous function. I do not think that's worth it; that's a much larger ABI transition than the one that's currently proposed.