I made a little test system for the 5 fireflies I just soldered. I made them run and I noticed a strange behaviour : they tend to oscillate by groups, so there IS coupling, however they usually make 2 alternating groups.

I installed the circuits inside a cardboard box, with alu tape over them to increase reflection of their own light.

The coupling is quite strong since a few cycles are enough to sync them... except that they tend to split into 2 groups (including one circuits that jumps from one group to another, maybe one of the parts is not the right value, or differently lit ?). Anyway, when they sometimes all turn on in sync, it's pretty powerful :-)

Then it struck me: the light sensor is active during one part of the cycle only...

I'll try to add another LDR in series with another 1M resistor, to cover another phase. I'll have to try different configurations to find the one that works best...