If your great-great-grandfather said one thing, and family members said something else, I'd suggest that you believe him instead of them. After all, he was the one who lost an arm in battle. For what it's worth, (1) there is no Thomas Dobbs on roll with the 1st Georgia Cavalry, and (2) Thomas Dobbs would have called the battle Sharpsburg, not Antietam. Antietam was the name used north of the Mason-Dixon line.

Despite the loss of his right arm, Thomas Dobbs remained on roll with the 23rd Georgia Regiment.

Click on the battlefield tablets to read each one. Note the highly-detailed maps of the battlefield --
http://aotw.org/officers.php?unit_id=614