That's not really a BEC of photons in the strict term of the BEC meaning.
Remember, a BEC is formed by a macroscopic population at ground state in thermal equilibrium.
This is more a statistical trick where you slow down some photons by dumping and reflecting them in a liquid dye, thus allowing them to reach an uniform temperature among the group. In reality they attain an average
thermal equilibrium, but they have no macroscopic population and not at ground state.
This is more a new form of matter, a standing wave, or a new form of laser if you want, but not a BEC condensate of photons.
The main issue in attaining a BEC of photons is that when you try to cool down photons (i.e. you attempt to reduce their energy) all what you will get is that you lose them.
Because photons are not particles, they do not have different state of energies like particle do, they cannot jump from one state to the other.
Photons are excitations, they get generated or absorbed WITH different level of energies.
And this is exactly the neat trick they use in this research to try to create an uniform emission of photons all at the same energy level.
But again, no BEC of photons here folks.