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 (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.