There is ABSOLUTELY nothing wrong with using a Dremel Tool to do an action polish job with. (Every gunsmith I've ever known - and I've known a few - used both fine India stones, and a Dremel-like power tool for his trigger jobs.) In fact I consider using Flitz and a Q-Tip to be a complete waste of time for any, 'quality' Glock trigger job.
The real mistake the OP made is that he didn't stay away from the edges! NEVER TOUCH AN EDGE WITH A DREMEL TOOL. Too bad because while a ruined trigger bar ain't no big deal, Glock firing pins are really expensive!
Next time, work barehanded so you can gauge heat buildup in the piece. Don't, 'touchdown' for more than 5 or 6 seconds at a time; and STAY AWAY FROM THE COMPONENT EDGES.