They like using the word 'sell out' a lot in their releases, and saying things like 'certain sections are sold out' or the match is 'on track for a sell out' a lot, which is a bit of a double edged sword.
On one hand it can make an event seem more popular than it is and encourage people to get a move on and buy tickets, on the other it can see people who were planning to buy suddenly not bother as they think the event is sold out or all the decent seats are gone. Unfortunately it looks like the later has happened with the opener as the lower bowl was pretty much near capacity 2 weeks ago (roughly 35K).
It's not impossible to move those last seats, but never great when momentum stalls, still we should hopefully see 40K+, which as Perth Red said, is a strong opening crowd and will look good on TV with the lower bowl full. Hopefully as well we can get close to the 45,032 we got at Millenium Stadium for 2013 (which in fairness was also a double header and also included the blockbuster fixture of Australia v England).
Whilst not the opener, positive that England v France has now less than 5,000 tickets. That means that match is over 23K.