Here's the thing. There is no highest dexterity among them because they are identical creatures. There is no tie because there was only a single roll for the entire group. So when do they go? At the same time.
This can only happen with DM controlled creatures, though, so it neatly avoids confusion between player characters since they always invoke a tie breaker (their choice or die rolls).
"If a tie occurs, the DM decides the order among tied DM-controlled creatures..."
4 goblins have the same initiative count, for example, but the DM normally just states what each is doing in the order the DM determines just like a tie. I don't think going on the same turn is the same as precisely the same time. Generally a reaction to time it with a readied action is required for that.