I agree that the numbers should just speak for themselves

Cutting 70°C in half is by definition -101.5°C

I'd argue here that no one would make this leap nor mental calculation, and most people would just divide X by 2 and gauge what the resulting Y is based on their familiarity with the weather.

it requires the consumer/reader to make a number of inferences or assumptions which isn’t a good method of communication in general

They still have to make these inferences to understand whether or not 70 to 35 is a remarkable feat or not.

If it's 30 / 2 = 15, people would think "Huh, 15 is pretty cool compared to room temperature ~ 20ish , that's significant". If it's 90 / 2 = 45, people would think "Huh, both 90 and 45 are pretty hot, but it seems like a meaningful reduction nonetheless."

I dunno, maybe I'm overdefending this