[dang]

YC doesn't do memos, and I doubt that partners have been telling founders to do this. Some of the lingo around startups can be fashion-driven.

Personally I find the term "founding $employee" a bit oxymoronic and I tend to take it out of job ad titles on HN when I see it.

FWIW (<-- I am not a startup advisor!) my answer to your question is that any engineer who is inclined to "just build it for themselves and retain 100% equity" should do so. The only reason not to is if you don't want to, or have reason to believe that joining someone else's startup will be better for you in the long run.

Sometimes engineers want to work at an existing startup for a while, in order to learn things before starting their own. That seems legit to me. But you can also found your own startup without doing anything like that. Plenty of founders do, and YC is happy to fund them.