Not a bad question, and they attempt to address it in the paper with their multi-sample calibration: It is highly unlikely that oxygen could have been injected

Not a bad question, and they attempt to address it in the paper with their multi-sample calibration: It is highly unlikely that oxygen could have been injected into halite inclusions without evidence of morphochemical disruption, and we must conclude that Cretaceous atmospheric oxygen was most likely higher than present-day levels. The tests performed on the modern, Messinian, and Cretaceous halites support halite’s robustness as an atmospheric oxygen archive.