They could have had an electric hot water tank that fed both units at one point, and then went to on demand hot through the boiler. The paperwork might just be old. Some older homes that have been turned into rentals have some crazy things going on. My panel feeds the on-property garage sub panel for example. I worked out a deal with my landlord over this, but technically it's a no-no.

It could also just be a regular old boring error, and only exists for units with electric hot water heaters that are included for some reason, like heat / electricity included units.

Basically, you got on-demand hot through oil, it is what it is.