Lindelof has already contradicted Ridley's statement by stating "any sequels would tangentialize EVEN FURTHER away from alien", not get closer as Ridley is inferring. This is in itself doesn't make sense though because that would require Prometheus to have deviated away from Alien in the first place, in which it does not, AT ALL.
Also Ridleys statement means Prometheus IS a Prequel, akin to phantom menace in relation to new hope.