It all comes down to the power efficiency of their chips
Results look disappointing for the Pixel 8 Pro in this test as it lands last for battery life and does a bit better than the iPhone 15 Pro Max in terms of charging speed.
About how they perform their battery life test:
This is done by using a light meter to calibrate their displays to 150 nits, then running a script that simulates web surfing over 5G cellular connectivity. This is done until their batteries are depleted.
I don't doubt those results. The Tensor isn't particularly efficient, but for what it's worth I've never had problems with battery life. I always have plenty remaining by the end of my day, and that's the measure that really counts to me as the user.