There are very easy, well documented ways to defeat an account password. Many of them are even documented on Apple's support site, like this one:
http://support.apple.com/kb/HT1274 . The only way to effectively protect against data theft is to encrypt the data.
Having said all of that, It's quite possible that the Apple tech was indeed blowing smoke at you, and the the repairer never guessed or even reset your password at all, or even accessed your account. If you want to test Mac A's superdrive, It's quite easy to hook one Mac A up to Mac B with FireWire and test out the superdrive from Mac B. This does not require access to any files on Mac A's hard drive.
PS, when sending a Mac in for repair, there's always the chance that the repair technician will screw up and destroy all your data (ie drop the hard drive). This is why the fine print of every repair contract says "repairer is not responsible for any loss of data." So it is vitally important to make a complete backup of your data at home before sending the Mac in for repair. And since you have a complete backup at home, you might as well reformat and zero out the hard drive before sending it in. That way no data will be vulnerable to any snooping repair techs.