Yes, it's simultaneous turn-based and mainly designed around the idea of multiplayer. Each player plans 5 seconds, then it plays out. That's why you see a bit of a jerk every 5 minutes as suddenly there are new plans, because you didn't expect the grenade to be off 1 degree and hit you in the face.
The planning is very inticrate. You can differentiate between "do this" and "do this, unless you see a new enemy, then shoot it" and "unless you see a new enemy, then run for cover". If that means they run into the line of fire of someone else, so be it.
Of course seeing an enemy depends on what direction you're looking. Do you want to spend a precious half second to look over your should or not?
There's also the option of "shoot in that direction" or "shoot in that direction, if you see an enemy" or "hold fire" or "fire at will", each with their own details.
But that might give you the idea that there's anything strategic about it. Mostly it's just levelling everything with grenades (marines), missiles (mech) and warp explosions (greys), with the swarm throwing absolutely crazy numbers and speed at the enemy and hoping something sticks, which it probably won't. Everything that dies leaves a corpse that the swarm queen can eat and poop out new eggs. Difficult game plan.
At it's best, you're keeping the enemy busy at one point and sending one or two flanking parties to hit him from behind, You can see some of the more advanced play at 6:48
If it wasn't crazy expensive back in the day (like 20$ a month?) it might have been more succesful. It's what the Gollop brothers went on to make after xcom.