Can one engineer really do the work of a whole dev team?

Yes, when the engineer is senior and the tooling is modern. That combination changed what's shippable in a week. A team of one or two now covers what used to take five, without the cost (a senior US dev runs about $300K year one once you include benefits and the recruiter fee), the wait (52 days on average to fill the role), or the churn (40% first-year attrition is normal at a startup).