The difference isn't so much the number of teams with a "credible chance" of winning. The big difference is that barring a miraculous turnaround in form, New Zealand won't be evens or better to win the whole thing as they (IIRC) have been previously. In a book with no overround, NZ being evens means that the rest of the field only have a 50% chance of winning to share between them. A more even distribution of the probability of winning is pretty much how I would define "most open".
I'm pretty sure New Zealand aren't usually evens or better to win as you state. I've been gambling since 96 and I don't remember it being that way, except in 2007 I remember at some point the all blacks were paying $1.40 to the dollar (noting because of the margin the bookies make this doesn't actually equate to a 1/1.4 chance of winning; the chance of winning will actually be higher).
But your point remains, whether the all blacks are typically 40 or 60 per cent likely, tightness in terms of "genuine contenders" should be measured by the variability of the odds - by whatever metric you feel is right- eg the number of teams with greater than x per cent chance of winning. And because the all blacks usually have much higher odds than they do this time, the odds left for the rest will be more this time. 99, we might have been favourites but I can't imagine we'd have been over 50 per cent given we had lost 5 in a row to springboks and wallabies the previous year. 95, not so sure but we hadn't been all that dominant in the few years prior - our record against South Africa, france, Australia wasn't great. Before that I dont know because I didn't watch rugby back then.
Edited again below because misunderstood how the reported probabilities should be read- thanks redruth
But note, none of the below account for the bookies margin so actually the probabilities should be less.
Edit: out of interest, just looking back at a couple of pre World Cup articles, , the all blacks were considered 40 per cent likely in 2019 (to be fair that's way more than I expected), and 45 per cent in 2015. 2011 they were 53.. Couldn't find for 2003 but I suspect both england and New Zealand were in the 40s with australia around 10 percent as the third most likely to win.
Edit again: just looking at bookies odds ), and again just using the first source I can find for each World Cup, fifth most likely to win for 2023 has implied probability of 12.5 per cent, compared to 8.3 in 2019, and 9.1 in 2015. Ireland are currently fifth favourite to win 2023 btw.