No, they don't. The phrase "they act at the same time" is referring to their entire turn. It does not mean their Actions are all keyed to take place at precisely the same moment.My position is discussing RAW.
The rules specifically say they happen at same time.
This means, as the DM, if I have a group of monsters, I can break up their moves and Actions however I like, and I don't have to do it the same way from round to round. It does not mean that all Actions are keyed to be exactly simultaneous, down to the nanosecond, because RAW, D&D rules abstract away natural, normal activity, not video-game-like animation frames.
We cannot both draw guns and perfectly time our shots so that the bullets strike the same target at the same instant...let alone do so involuntarily over and over...let alone have a whole infantry regiment do so. So, no, doesn't pass the common sense test.There is nothing that prevents two DM controlled creatures from taking their turn at the same time. The precedent is literally written into the description of initiative. It even passes the common sense test, since we can both draw a gun and shoot each other successfully.
What you consider RAW and what's actually RAW are two wildly different things in this instance.You don't have to agree with me, but I consider it RAW.