Correct, the aeb-bracketing is disabled in the M mode. Since you are setting shutter speed and aperture manually anyway, the reasoning is probably that you can also bracket manually. Easy to do if you want to but i.m.o. not really necessary because the histogram in shooting mode tells you more accurately than the exposure scale when your exposure is best.
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-Better a small camera in the pocket than a big one on the shelf --
AEB should not be available in manual since you are controlling the f-stop and shutter speed having overriden the cameras ability to automatically control these.