Especially because they seem to test it by making it predict history, but we don't have anywhere near the same amount of historical data. So what are you testing, really? Just that it accurately finds average temps?
Climate models as far back as the 1970s have predicted the temperature changes in the time from then to now -- those are not predictions that are coming from hindsight, but rather predictions where their effects were observed over the last 50 years.
If you are unfamiliar with climate sensitivity, this is a good place to start: https://skepticalscience.com/climate-sensitivity.htm