Added the first part of the sentence missing from your example. Prolonged exposure to an unhealthy stressed environment can create physical changes in the brain and body. In this case, chronic stress which had such adverse effects it caused a forced medical leave.
> ...from the stress of being a software engineer
"My two years at Google were spent perpetually stressed, acting as an unofficial PM, helping run meetings and document decisions while dealing with sometimes hostile colleagues." - I can't speak for others, but this is not the 'normal' stress of being a software engineer that I know. That has all kinds of red flags. I have never been asked to perform the duties of an assistant/secretary or help organize a meeting that was not mine. Nor have I had to juggle the actual normal stress of my software job on top of having an unofficial/unpaid PM role (itself a full time job), while also having to deal with hostile colleagues. And, it seems they were not the only ones having to deal with some of this.
https://www.health.harvard.edu/mind-and-mood/protect-your-br...
https://www.metabolismjournal.com/article/S0026-0495(05)0003...
https://www.sciencedirect.com/science/article/abs/pii/S03064...