Frankly, I think you've brought up the speed of light as a distraction.
More interesting than whether they are moving at the same speed, is whether they are thinking at the same speed, particularly, whether they think with a common tick. Since time appears for the most part to be continuous, that's not going to get you a tick, nothing for the ants to synchronise their clocks to.
If being in the same frame of reference was enough, then you could just put two computers in the same room and call it a synchronised system.
More interesting than whether they are moving at the same speed, is whether they are thinking at the same speed, particularly, whether they think with a common tick. Since time appears for the most part to be continuous, that's not going to get you a tick, nothing for the ants to synchronise their clocks to.
If being in the same frame of reference was enough, then you could just put two computers in the same room and call it a synchronised system.