(disclaimer: I've worked at two mega corps and a startup and generally favor my experience at the second mega corp so far)
I think your example points out a factor, but not the only factor and probably not the biggest factor. You can also say that startups don't live long enough for their code to grow organically from their original, pristine design and develop deep seated code "cancer" like mega corp code does. Also, even without turnovers, some features just go through enough code modified by enough engineers that Murphy's law happens and things break in creative ways that E2E tests catches or could have caught. And finally, just because I remember my code today doesn't mean I'll remember it in a year or two... I don't assume that my coworkers have perfect memory either.
I think your example points out a factor, but not the only factor and probably not the biggest factor. You can also say that startups don't live long enough for their code to grow organically from their original, pristine design and develop deep seated code "cancer" like mega corp code does. Also, even without turnovers, some features just go through enough code modified by enough engineers that Murphy's law happens and things break in creative ways that E2E tests catches or could have caught. And finally, just because I remember my code today doesn't mean I'll remember it in a year or two... I don't assume that my coworkers have perfect memory either.