It was just very aggressive... the peak charge voltage was at the absolute upper limit for LiPo cells and they used an equally aggressive charging profile to charge it quickly. So just the act of charging the phone to 100% put the cell under a significant amount of stress, more so that most phones.
Frankly, as someone who plans on keeping the phone for an extended period, I'm more than happy to sacrifice charging speed for longevity. I had to swap out the batter on my 6P at about 15 months as it was down to about 85% capacity (even with my somewhat conservative charging practices).
Unless there is a big shift in battery technology, I wouldn't expect to see faster charging phones from Google. They clearly have made battery longevity a priority (it was one of the marketing points when the Pixel 2 launched). It remains to be seen if this pays dividends in real world usage, but on paper, less charging stress should mean longer cell life.