A simpler thermometer may have a single IR receiver. In that case its output can be far from truth without an emissivity calibration.
Better IR thermometers have 2 or more IR receivers for different wavelengths and they compute the temperature based on the ratios between them.
In this case the measured temperature no longer depends on the value of the emissivity, but only on the deviation from the behavior of a gray body. With an increased number of IR wavelengths that are measured, it becomes less likely that a calibration could change the results, but cheap thermometers are unlikely to use such a sophisticated method.
A simpler thermometer may have a single IR receiver. In that case its output can be far from truth without an emissivity calibration.
Better IR thermometers have 2 or more IR receivers for different wavelengths and they compute the temperature based on the ratios between them.
In this case the measured temperature no longer depends on the value of the emissivity, but only on the deviation from the behavior of a gray body. With an increased number of IR wavelengths that are measured, it becomes less likely that a calibration could change the results, but cheap thermometers are unlikely to use such a sophisticated method.