Indeed. My B2B charger detects the current going down as an indication that the battery is fully charged. That's all very well, but it has no way of knowing whether that current is going into charging the battery, to charging your laptop or powering the fridge. For that reason, my three way AES fridge is powered from the starter battery.Many State of Charge Battery Monitors, such as the Victron BMVs, will use the level of charge current as a way to tell if the Battery is fully charged.
I'm guessing that the complex Victron units have a shunt that can record all the current in and out of the battery and so they can tell that 3A is going to load and 4A is charging (or vice versa).
Most solar controllers are not wired to use their load terminals, and few B2B chargers even have load terminals.