Did those units shoot? What happens when there is a few thousand projectiles in the air, fired by those 1000 units? What about map deformations? What about map wide physic simulations, like springs tsunami water sim or the lava flows of a volcano, changing directions?
None of this should matter as long as the algorithm determining randomness is deterministic.
The bottleneck is player input which is the most overestimated bandwidth stat in gaming. It's mouse movements and a couple of keys strokes per second. Top Starcraft players are in the 300 actions per minute range, that's still just 5 per second.
You don't need to track every projectile, you just need to know that player 1's unit 33 started shooting at player 2's unit 45.
You can encode that very compactly. 2 bytes for each unit ID, source and destination. So 1000 units would be just around 4K, if they all start shooting at the same time.
After that, you can rely on that in most RTS games how units shoot is deterministic.
