The sort of person who grows a company from Show HN -> Series D is a very different sort of person than the sort of person who can join as a C Level exec.

There's more overlap than I would've suspected - a lot of the Director/VP types that I saw Google hire from the outside (as opposed to promote from within) were folks who had previously founded a company, sometimes up to 4 companies.

Agreed, but that's different again. Once you've done a job like be the CTO or CEO or an exec role at a high growth startup (like Segment) then getting into a role as an exec at a larger company.

Wouldn't getting to the point where you are a C-level at such a company be pretty stressful as well?

Usually the folks who join a (successful) company as a C-level exec at year 6 are those who previously stressed out for 10+ years, either as a founder or working their way up through multiple successful projects at a big company.