My observations align with this, that it’s probably not as available as ammonia. However, it might still be a good plan. The issue I have with ammonium bicarbonate is the amount I have to dose to get measurable nitrate. If I am using nitrate as the indicator, my corals darken by the time I see nitrate steady in the water. My choice is to dose less ammonium bicarbonate and accept zero nitrate or add something to keep nitrate above zero.
By mixing in urea, I can achieve a low level of nitrate with a lower dose of ammonia.
I could accomplish this another way, by just dosing a little sodium nitrate with my ammonium bicarbonate, but so far urea doesn’t have any observed downside in my tank.