That does not make sense to me. Aside from him dosing a mineral acid (effectively dosing negative alkalinity), there's no normal process that depletes that much alk and not calcium in a reef tank in 9 days (unless the tank is actively cycling by adding ammonia repeatedly).
AFR and kalkwasser have exactly the same alk to calcium balance. Yes, both are balanced to a bit more calcium than alk demand, but those effects show up over months, not days. Regular water changes should suffice to eliminate any such concerns. They were in my reef tank using kalkwasser for 20 years.