That is not correct. Yes you can pronounce it "dom" for de but you can say "de" for de as well. Dom can be used for both "de" and "dem". The same goes for the dem, it can be pronounced as dom or dem. You should always write de or dem and never dom when you write formal Swedish according to Svenska Akademin. Example. Dom har dom should be written De har dem. Although the dom is used more in conversational Swedish you will still come across the de pronounciation.
Dom is also the word for verdict, but is pronounced differently.