In the frame of reference of the ants, all clocks agree with one another. This means that timing is global within their FOR. There is no problem in determining simultaneity because there is only one FOR in question. And even in multiple FORs, their relative speed would have to approach that of light for any ant to notice a difference between local times. Deny at your own detriment.
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.
Ants work together to form an efficient parallel system, where is their global clock?