Diego did not know why Jin was not there to aid them. Given the person who had been recommended by Leader Yuan was supposed to be more than capable of clearing this entire mess, he instead decided to give them advice on how to eliminate the threat.
Regardless, Diego immediately relayed the information Jin had given him to the rest of the group and they managed to regroup and follow Diego's latest intel to fight against the Y'm-Bhi. Soon, they realised that what he said was true. The fight became easier and they eventually managed to have total battlefield control of the area.
This even gave Sir Diego some time to put up a proper fix to the dimension tear. While some of the Vatican Guards asked if this was the right decision since some of the Wang Liangs were in there as well. It would have been better to lure them all out and kill them.