Prohibition
Did he just get desperate, and in the throes of DT's, start stumbling around the house, drinking anything chemically-smelling that he could find? Rubbing alcohol smells kind of like liquor, I guess. Why didn't Mr. Alcoholic try drinking the bleach, or the Lysol? Maybe he did, and we just never heard of it.
When Prohibition was enacted, did the major newspapers run a feature (in the "Life" section) about which household products were least fatal? They're all pretty fatal. A few mouthfuls of methanol is deadly, and no true alcoholic is going to stop after the first swig.
Maybe it was spread by word of mouth. Someone drank a whole bottle of antifreeze, didn't die, and suddenly everyone was doing it. That doesn't explain where he got the idea to drink the antifreeze in the first place. Maybe he was an alcoholic doctor? Maybe he was just a pioneer, one of those half-crazy people who goes where destiny takes him. It is thanks to unsung heroes like him that we know: Antifreeze is slightly potable.