Why does a MacBook throttle without a battery?
The CPU and GPU can draw more power than the AC adapter can provide as the battery serves as a capacitor and reserve. It smoothes out the voltage when a surge is needed. If the system over draws, voltage drops a little and subtle computing errors happen. If the voltage drops too far, the system shuts down entirely and abruptly.

To be reliable and fast, the battery is needed and the system slows down to preserve the correctness of the function. This is a very well tested design, so it’s not an accident that this happens and it’s not for some small reason. This can be measured with oscilloscopes and quantified precisely. 

All the measurements you are doing are time averaged. You’ll want to look at millisecond to second level time scales to see this voltage compensation happen.

Apple engineers work from Intel specs on voltage drop curves and tolerances to meet the needs of the power handling on the Intel side as well as manage the battery and charging circuitry and also build in tolerances for aging gear and adapters.