Going from extreme heat to extreme cold instantaneously is bad for anything (computer component or otherwise). Your laptop components take time to transition from one extreme to the other. You can take heart in the fact that while your CPU may reach 70c while gaming, as soon as you turn it off, it begins to cool rather quickly. It should also be noted that while in your backpack, it does develop an envelope of warmth around it which helps in tempering the temperature changed. Also, it's only the CPU that reaches 70c. The fact that it's enclosed in a laptop's casing also protects it.
I would say that between the time you shut down your laptop and the time it takes to get bundled up to go outside, the laptop has sufficiently cooled to where it's safe.