Hi, the author here. I'm using voltage sag to describe how the voltage drops as the load increases. (This has nothing to do with the presence of a battery or not.) Chargers normally are designed to output a constant voltage until the charger hits maximum power. But with the iPhone charger, the voltage is 5.2 volts if there's no load, and steadily drops to 4.6 volts as the load increases. If a device is expecting 5 volts, this could cause problems. Also, charging times could increase since the voltage is lower.

Generally, if the voltage drops under load like this, it's a sign of a poorly designed charger that doesn't regulate well. But since Apple's charger appears otherwise well-designed, it's a bit of a puzzle. One possibility is that maybe the exact voltage input to the iPhone doesn't matter, and the designers knowing this didn't care about the voltage sag. Another possibility is for some unknown reason they deliberately designed the charger this way.