Thermal scanners will detect people with a high fever - beyond the normal human variation. So they have a certain true positive rate when used like that, and low false positive. However they will also have many false negatives as not everyone developes a high fever immediately upon coronavirus infection (or ever). Lowering the alert threshold to below the top of the normal human variation has an obvious problem of too many false positives.
Used to detect high fever, they will find some sick travellers which has an obvious benefit. They cannot detect all infected people (and nor can anything else).
Low rates of detection by those scanners may well reflect that in practice people with high fevers rarely travel, as they are obviously ill. That doesn't necessarily make them useless. It does make them insufficient for anywhere near complete assurance that no infected people travel.