> If you start with two disconnected cell groups, how do you know when they collide?

You create 'dead' cells in between that connect the two populations.

That's also why you can't remove them. Remember, this is a simulation of a fabric, the dead cells not being there all the time is an optimization, in a real fabric all cells would exist all the time.

The async component is the fact that there is no global authority supplying a clock, each 'peer' is on its own from the moment of power-up.