I refer to the fpp2 testbook: 9.3 Forecasting | Forecasting: Principles and Practice (2nd ed)

In this example, the electricity demand and the temperature series are both non-stationary. Elsewhere in the book, and as we know commonly, the advise is to either check for co-integration, or to use differencing to make the variables stationary.

Hence, isn't doing the regression a problem, as it is essentially based on spurious regression? How do I justify this regression?

^{Referred here by Forecasting: Principles and Practice, by Rob J Hyndman and George Athanasopoulos}