I believe ethanol is not actually often given as an antidote for methanol poisoning in modern times. It does work as a competitive inhibitor of alcohol dehydrogenase (i.e. occupying the enzyme to convert ethanol to acetaldehyde, slowing the conversion of methanol to formaldehyde and on to formic acid, which is not eliminated quickly and causes metabolic acidosis) - allowing the methanol time to leave the body through excretion, and limiting formic acid levels. However, other drugs like fomepizole also inhibit alcohol dehydrogenase with lower toxicity than ethanol.