So, was their continuation of working to (a) save more money, (b) earn more pension credit, (c) a+b, (d) something else?

BC, with social security, waiting until 65 to collect is a meaningful bump in the benefits, and waiting a little longer even more. Holding out until 70 — and, again, with a $20,000 maximum income — makes the individual benefits $13,236 per year — times 2 that’s well above the poverty line, but still not very much (but pretty good compared to $20k and 40 hours a week — especially adjusted for taxes).