I don’t think that is true. I read it as the maximum, not the slope. The confusion is from the use of higher instead of faster. Faster would be the rate of change while higher is absolute value.

> We know for sure that the average temperatures have risen higher in the last 40 years than they have ever before.

This could be a year over year increase in maximum, with a very small slope too

The post in question writes "have risen higher", which to me reads as if it's saying that the temperature, as a result of rising, is now higher than ever before, not that the rate of change is higher. I would expect something along the lines of "have risen faster" if we were describing the slope of the graph of temperature over time.