That’s just the entry time. If you want to do the Verger tour, you sign up when you get there. I think the first started at 10:30 but double check the website. You might still have a line so getting there by 9 and walking around before the tour starts is fine. If you want a guided tour that starts at 9 though, you’ll have to check with other private companies.
Edited to add- people were still waiting in line to get in even if they already have a ticket because there were no signs saying otherwise and we were obedient tourists. It also gives you a chance to buy a tour ticket at the door instead of inside. To make the 10:30 tour, I’d get in line by 10 at the latest just in case the line is long. I don’t recall people having a problem joining at the last minute if you get inside right before a tour starts.