DriveThruRPG is the same company as DMsGuild (formerly DNDClassics), and the former RPGnow. They have the same inventory and you can login to either with the same account. As far as I know, DM's guild is the only WoTC-sponsored site that sells PDF classics.
Early in 2016 DM's Guild posted a request for scanning and remastering some classics. From the post:
To that end, together with Wizards of the Coast, we have been digitally re-mastering hundreds of classic D&D titles and offering them through DriveThru and DMSGuild.com. We still have about 600 more classic D&D titles to go, and we need the help of D&D fans to get them all.
The original post is no longer available, but the archive link above may still answer some of your questions. For example, cover and boxes are scanned at 24-bit depth and 300dpi, while interiors at 8-bit depth and 300dpi.
Basically, your assumption is spot-on and we're talking about high resolution scans of original prints.
While browsing a product in DMsGuild, there's a bar on the right side containing the Product Information. Under "Format", you should see either: Scanned image or Original electronic format.
Clicking on the info icon will display the following:
These products were created by scanning an original printed edition. Most older books are in scanned image format because original digital layout files never existed or were no longer available from the publisher.
For PDF download editions, each page has been run through Optical Character Recognition (OCR) software to attempt to decipher the printed text. The result of this OCR process is placed invisibly behind the picture of each scanned page, to allow for text searching. However, any text in a given book set on a graphical background or in handwritten fonts would most likely not be picked up by the OCR software, and is therefore not searchable. Also, a few larger books may be resampled to fit into the system, and may not have this searchable text background.
For printed books, we have performed high-resolution scans of an original hardcopy of the book. We essentially digitally re-master the book. Unfortunately, the resulting quality of these books is not as high. It's the problem of making a copy of a copy. The text is fine for reading, but illustration work starts to run dark, pixellating and/or losing shades of grey. Moiré patterns may develop in photos. We mark clearly which print titles come from scanned image books so that you can make an informed purchase decision about the quality of what you will receive.
Original electronic format
These ebooks were created from the original electronic layout files, and therefore are fully text searchable. Also, their file size tends to be smaller than scanned image books. Most newer books are in the original electronic format. Both download and print editions of such books should be high quality.