It is used as an excuse by quack doctors (including naturopaths and "functional medicine" practitioners) to diagnose people with hypothyroidism even if all the blood tests come back normal. Here is a good summary: https://hormonesdemystified.com/basal-body-temperature-and-y...

As for the paper you linked, a correlation of -0.013 degrees centigrade is too small to be useful as a diagnostic tool.