It's so right that it's absent in the subsequent parts.

This is by no means an indicator. Parts 5 and 6 are aimed at the masses and are mostly infantile and silly. Are 4 resources really an expansion of tactics? O_o
Simultaneous attack makes the game more complex on one hand, and on the other hand, it reduces, say, the importance of speed.

Yes, with a flag wave "one, two, three," they probably rehearsed at home with each other.

It's strange, but with the dwarf and phoenixes, the imagination kicked in, but not in the opposite direction. Simultaneously means that both attack, not necessarily perfectly synchronously, but that's it. This is generally the case in any battle (for example, with infantry and cavalry). But it's definitely not about realism, but rather about new tactical variations.

What kind of tactics are we talking about if they die like flies from this?

This only characterizes your ability to manage troops. Although, in essence, in part 3, for example, tactics such as "taking" the hit with a left or "sacrificial" unit, or even with single units, are also used. So, you don't necessarily need to come up with anything radically new. It's just that before, it was probably necessary to "take the hit" less often.