He was quoted as saying going without amphetamines for a month set back mathematics a month.

Which he did to win a bet on whether he could go without stimulants for a month.

I read that he complained bitterly the whole time and proclaimed that the bet had cost Mathematics immeasurably. I wonder how true that is.

The complaining part is probably true—amphetamine withdrawal will make you extremely irritable, and moreover he probably felt like he was being held back from his full potential.

Probably. He seems like an endlessly fascinating character to me. He's almost tragic in a sense, but I also can't help but thinking that he probably felt more fulfilled than most people.

The story in which this background is embedded is about Erdös and Guy getting a cup of coffee, so it doesn't seem unreasonable, and is entirely factually accurate, to mention first Erdös's reliance on coffee to fuel his inexhaustible mathematical energy, and then to acknowledge other drug abuse.