"But considering Wales were No. 10 when Warren Gatland took over in 2008, it's a pretty massive achievement.
"We may have not beaten New Zealand since 1953, but we're finally ahead of you in something in rugby."
The proud moment for Wales has been tempered by bad news on the injury front, with first five-eighths Gareth Anscombe and No. 8 Taulupe Faletau ruled out of the World Cup, which starts in four weeks' time.
"A lot of people have got issues with the world rankings. It looks a bit odd…how can New Zealand drop in the week they beat Australia 36 – 0?," Thomas said.
"But it's not about one day, it's an accumulation of results. Wales have won 15 of the last 16 games they played. In that time they've beaten South Africa and Australia which New Zealand hasn't done on every occasion. It's a reward for excellence."
Wales will be expected to emerge from their World Cup group, which includes Australia, Georgia, Fiji and Uruguay. But their record is very disappointing since finishing third in the inaugural tournament, in 1987.
Thomas said a lack of depth was often Wales' undoing in the World Cup, and Gatland had addressed that situation by building depth in every position.
But certain players including captain Alun Wyn Jones and No. 10 Dan Biggar were almost irreplaceable.
Biggar and Anscombe had been used as a tag team, with the Kiwi providing the early thrust and Biggar guiding Wales home in games.
Wales were now very thin at No. 10 because of Anscombe's injury, and he was also set to figure at fullback in the tournament.
The injury situation meant the two remaining warm-up games against Ireland were a double edge sword.
"There is now a lot of burden and responsibility on Biggar but the one thing you would say about him is he is the ultimate competitor, hugely resilient, hugely brave, and makes the absolute most of his ability," Thomas said.
"Every extra game is an injury risk…but traditionally Wales are slow starters in campaigns. They are a team that builds so the warm ups are very beneficial so long as we stay injury free.
"A week later they were a very different side (in the second game against England) with greater physical intensity, the work at the breakdown, ability to slow the ball down.
"England had a monstrous pack, one of the biggest I've seen come to Cardiff…but they really got on top of England in that game."