You can have a low grade fever (up to about 39 C or 102.2 F) in any number of conditions that aren't contagious and don't represent health risks to yourself or others in the context of flying. Examples include lupus, arthritis, thyroid problems, cancer, urinary tract infection and even stress.

An accurate thermometer might be a very sensitive but not specific test for untreated Covid-19 symptoms. Untreated because in most cases of infectious causes of fever, most over the counter analgesia (aspirin, Advil, Tylenol, naproxen) will lower your fever. But most healthcare workers will tell you that the problem with the handheld "gun" style thermometers is they are not very accurate - the best style of thermometer would be placed under the tongue, or in the rectum in infants/very young children. The way thermometer guns are calibrated, if you hold it too close you might get an inaccurately high reading, and if too far away, too low a reading. While traveling in Asia I've been told my temp is in the 35-36 C range using a gun-style thermometer, which is quite low and almost borderline hypothermic, likely due to poor measurement technique or a poorly calibrated thermometer.

I'm not saying that taking passenger temps has no value, just that the interpretation of an elevated temperature (especially if mild), and the accurate taking of a temperature, is probably beyond what can be expected of a gate agent at the time of boarding a flight. It's more useful for screening if someone has a moderate/high fever (>39 C or 102 F), although someone with a temp that high often feels like crap to the point where they're using analgesia and may also generally look quite sick.