The ABS doesn't allow the wheels to do anything except stop
The Automatic transmission has an ingenious thing know as a torgue converter which senses engine speed relative to actual drive shaft speed and adjust everything accordingly. An idividual with a manual tranny accomplishes the same thing with the clutch.
The amazing thing about the automatic tranny in its basic form is that it does not use sensors to accomplish what it does. When you hit the brakes you force the car to stop the converter unlocks the drive shaft from the engine allowing the brake to accomplish its job.
It is likely that an automatic transmission does brake worse than a manual because the converter does not completely disconnect the engine from the drive shaft at full stop. There is still forward motion on an auto. But the difference in the full stop is not perceptable. You should be more concerned with the weight of the auto compared to the manual if you are searching for the discrepancies in braking performance.
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