"I don't think there's any definites out of the Six Nations. England, Wales and Ireland are all pretty close to each other," said Johnson.
"The atmosphere in Cardiff when it gets going is very difficult to overcome and they were inspired.
"[But] the World Cup is six months away and in a neutral venue. It's a different thing."
Johnson, a former England coach, further downplayed the Six Nations result as a major World Cup pointer.
"England's performance against Ireland was probably the single most impressive performance of the tournament," he said.
"They then had a bad second half in Wales and blow a 31-point lead against Scotland.
"Wales were 16-0 down at halftime against France and won it on a pass that made you think 'goodness, don't throw that'.
"You can be the best team in the tournament and have a bad hour and a half and you're going home."
The All Blacks lost their invincible tag in 2018 after defeats to South Africa and Ireland, while Wales have now displaced Ireland as the world number two, but even despite that, Johnson doesn't think any team has sufficiently closed the gap on Steve Hansen's side.
But Southern Hemisphere teams have won seven of the eight World Cup tournaments and the All Blacks will be shooting for their third consecutive crown in Japan.