Blizzard owns the game, of course they have access to every database they need. They don’t need to consume any of the public APIs to have that kind of information.
For more details see this thread where people involved in gathering the same information pointed out how the AH changes would impact those datasets.
The general idea was to use the AH dumps to fetch character names, with their names you could get guild information and fetch even more names. That would cover all active characters that either belongs to a guild or uses the AH.
Since the AH does not include the seller name anymore due to the new data policy the ability to get that kind of information is very limited.
I guess you could achieve something similar by getting data from the in-game AH.
You might wanna join the discord server and see if anyone have other information to share on this topic.