Indeed, but note it usually works well enough, because most surfaces have high IR emissivity (in particular with organics like plastics, papers, woods, fabrics, etc.; soils and liquids as well).

The one exception you have to be careful is with unpainted metal (high reflectivity). If polished it can be so low that even calibration doesn't help -- because you'd need extremely precise calibration that isn't practical and because reflections and noise/other heat sources will pollute you readings. In that case a simple sticker (or thermocouple reading) would be more adequate.