Here’s my source. It’s the latter case, they use digital sales figures from the companies that provide them.
You raise a good point: if Larian aren’t sharing sales figures then it’s not possible to definitively compare them. I don’t think the 22M figure is very credible (as the other commenter said it doesn’t match up with the data we do have regarding player count/copies sold, and came completions) but even 5-7M copies sold sounds like it would place BG3 on the list. There’s enough bleakness in the gaming scene as it is, so I’m glad to hear it might not be quite as bleak as I thought.
On Steam, click your username in the top bar, and select “profile” from the dropdown menu. On the profile page, there’s an “edit profile” button near the top right corner, underneath your level. In the “General” tab, the field “real name” should be the second one down