Nero did not hesitate before launching his flames into the bones. He did not intend to freeze them. Rather, he wanted his flames to consume the energy that was trying to control the bones.
He did, however, retreat a little to create some distance between himself and the bones. The team also retreated while searching for the source of the sound. If the door to corruption had opened on its own, then they were in for a world of trouble.
But the door that opened wasn't the door to the final room, rather it was the one that led to the second floor. Another team had appeared, though they stopped when they saw Nero and the rest.
No doubt they too had orders to compete for the artefacts with the other teams, which meant that they weren't necessarily allies. Their first interaction would likely determine the nature of their relationship moving forward.