If you can see water in that pipe/trap, can you suck it out with a shop vac? If so, you then should be able to verify that it is connected to the sanitary/sewer by flushing a toilet somewhere and listening for the sound of water at that open drain.
If it were me, I would want to know if that is a trap in the ground and how difficult it would be to replace it with no trap. Cast iron traps are difficult if not impossible to snake through. So while the laundry sink is attached to it without issue, would a second fixture, albeit low volume overload it? Worst case scenario, you find it is over capacity and you switch back to the saniflo.
I think you have to determine the ABS is connected as above and that tells you what your risk factor is. If its connected, good to go with some risk. If its not connected, its a no go and wonder how the laundry sink functions.