You are correct - I was wrong. "Tiếng Việt" works here without fallbacks [1], though.
My theory is that the fallbacks are designed to work well for those popular fonts so it doesn't look off when the system falls back. The consistency in the width of the stroke seems to be the most important factor to my eyes. Here [2] you can see the mix looks natural, while in Georgia in dev tool it looks inconsistent. The screenshot is made on my Ubuntu computer.
