The oil pump drive is directly meshed with steel gears (both) to the crankshaft. The tach drive is a plastic gear driven from the same gear/shaft that turns the pump. Provided your tach was still working when the seizure happened the pump was still being turned.
Air is getting into the oiling system from somewhere. It often is the inlet supply banjo fitting. If the oil inlet in the tank was clogged the pump might suck air through one of its seals (or from the aforementioned banjo)
Have you been noting the rate of oil consumption during operation? Like resetting the odometer on a car each fuel up, and knowing how far a tank of fuel will usually go. If the mileage changes something is starting to go amiss.
The pumps usually are good unless corroded/clogged internally. Was the operating cable properly rigged/working?
IMHO, I'd disconnect the oil inlet line at the pump and confirm oil flows well to the pump before disturbing the system further to try to better find the starvation cause. If this shows little/slow flow you will know for sure there is an obstruction somewhere to the pump.