Stephen Ridley is a friend and a former teammate. He "moved on" from Matasano as well, to BankSimple. He "moved on" from Simple to full-time security research. Because he's a security researcher. Since he left Simple, he's been doing research and teaching classes on ARM exploitation. Because that's what he likes doing. How much opportunity do you think there was for developing ARM exploitation classes at Simple?

This happens all the time with very senior security roles at startups. People take them because they fit a career arc, but then realize that the day-to-day of those very senior roles just isn't as fun as research work. Since security research work pays like f'ing crazy right now, people bounce out of demanding corp roles pretty quickly: even in a very strong outcome at a startup, you might not give up too much money opting for "funemployment".

(Alex and Stephen are also good friends, from long before Simple).