How did the electrolyte boil off in the first place?
The typical reason is a very cheap battery charger that did not know how to charge the battery properly. Modern state-of-the-art battery chargers can charge lead-acid without any boil-off at all.
You hear about "maintenance-free batteries" for cars? That's a lie, the battery depends totally on the alternator being modern/sophisticated and knowing how not to overcharge the battery. If you use a maintenance-free or SLA battery on a garbage charger it will boil off electrolyte anyway, destroying the battery since there's no way to refill it.
The old way, back when battery chargers and alternator regulators had no silicon and were all relays that went clack-clack... you did boil off some, and you regularly replaced it with distilled water. That was part of the routine maintenance a garage would do when they changed your oil. Nobody does that anymore.
So whatever charger did this damage is a piece of junk. Note that a great many "12V sump pumps" are el-cheapo models mainly designed to trade money for peace-of-mind, rather than actually being reliable units that will last.
You can try refilling and recharging this battery. I would use a regular battery charger like one intended for an automobile battery.