It's going by this site:
https://www-aaaba-lp1-hac.cdn.nintendo.net/en-US/index.html
Pinshadow could likely explain it better, but basically, the URLs of the images for Mii costumes, characters, and other non-tournament events are numbered. To give an example, I looked up the images around the time of Terry's release.
Exclusive online offer before Terry's release: 051
Terry released: 052
Fighters pass available: 053
Ryo costume: 054
Nakoruru costume: 055
Iori costume: 056
Akira costume: 057
Jackie costume: 058
Version 6.0.0 update: 061
Version 6.1.0 update: 065
Version 6.1.1 update: 066
Photo contest: 068
Exclusive online offer before Byleth's release: 069
(59 or 60 was likely "Version 6.0.0 coming soon", which gets removed from the site once the update actually comes out)
The latest exclusive offer is 110, and
the previous numbered event is 103 (the Mario photo contest). (Steve's announcements were 102 and earlier, so the numbers aren't in chronological order) That seems to indicate that Sephiroth's announcements are number 104-109, which doesn't leave room for much if that's the case (it could be 111 and up, but then there's a big inexplicable gap between 103 and 110) - Sephiroth released, the version 10.0.0 update, and the Fighters Pass Available messages take up three of those six spots, and if there's a "Version 10.0.0 update coming soon! Your replays won't work!" notification (which there generally is), that's another spot taken up, leaving a whopping two spots for Mii costumes.