And who decides what “average” is?
The calculation of the average is agreed: it's the arithmetic mean. But, it's also worth mentioning that other statistical measures like median might be more appropriate.
The problem is that you can cause significant changes to any summary statistical measure by careful selection of data. Just choosing a time window can change it. The article's author accused Cruz of doing so, and then proceeded to do the same thing by choosing the 20th century.
Once you go back more than about 30 years, the source of data also affects the results. In the climate models, they do a lot of adjustment of source data (to "correct" it), and actually exclude data deemed "unreliable", sometimes because it is outside their expected range. But, what if the data is right, and your expectations are wrong?