Server Action steps - (a)synchronous?

In a thread earlier today (now closed), @George mentioned:

All Server Action steps ARE asynchronous!

I’ve always understood that server actions steps run in sequence - the next step only running when the previous one has completed - and this has also been my experience. So I assumed they ran synchronously. Are there situations where steps can’t be relied on to complete in this way? Hopefully this is not the case, and it’s just that I don’t understand the implications of George’s comment.