It's safe to assume that empty-net goals are almost always scored in games with a two- or three-goal margin of victory.
(It's not impossible, technically, to score an empty-net goal in a loss, or in a blowout win, but it's extremely unlikely). The Oilers had
23 two- or three-goal wins in 1982, so suggesting that Gretzky scored 30 empty-net goals is clearly unreasonable.
During the 1982 season, Gretzky scored less than 25% of the team's total goals. Even if we assume that the Oilers scored one EN goal in every single two- or three-goal win, he would be expected to have 23 * 0.25 = 6 empty net goals that season.