Yes - basically, high quantities of volcanic ash act as a much stronger binder than is currently used/available now, and the chemistry of the cement they used meant that as it aged it got stronger.

Tangentially - it's not that we can't make concrete that way, it's that for many structures we're building of concrete, building to last 100+ years is over kill, and would increase costs drastically for a building or structure that will most likely be torn down before it reaches its life expectancy.