Definitively external coupling failure, NOT bearing. No cooling required, because it's not a bearing surface.
Oil injection couplings basically use hydraulic pressure to drive an outer (female) shaft over and up a tapered shaft of lesser diameter. When the desired position is achieved, pressure is released and you've basically got an interference fit with very high friction and hence torque Tx ability. It's a very similar system to how propellers are installed on their shafts. The coupling is contained in a fairing to prevent water ingress.
It's pretty simple - which is why it's so surprising that it appears to have failed.