Technically speaking even that is not guaranteed.
Swifts actors are just not FIFO today. If a high priority task arrives and others are normal, it may get to execute before the others.
It was designed this way, in order to facilitate serving those high priority work as soon as possible. And even allowing an “skip the work, we no longer need it!” Messages to jump in front of the queue etc…
But yes, it means we just don’t — in the general sense of the word — have FIFO in actors today.
If all your work has the same priority, and all of this work has no suspension points then yes — you’d get FIFO behaviors, but it’s somewhat brittle.
I do think the requests for doing something better here have been heard, but so far priority was to get the isolation model without holes in Swift 6.