I strongly suspected that is what they did too. Got to wonder if they used the EXIF data to find him - linking photos of the pot and other social media/etc public shots, then used the fingerprints as parallel construction.
Well they also mention in the article that the drug dealer also sent bitcoin to a money laundering service run by Homeland Security, so his OpSec was probably as leaky as a noodle strainer.
Imgur strips exif by default. I agree it sounds like parallel construction.
The article mentions they compared his finger prints to those from the picture. How did they know to check against his prints? Sounds like they already knew who it was, by means that aren't admissible as evidence.
4chan is a bit special when it comes to government orders.
For example, they have a (heavily limited) de-facto warrant canary. Since damn near all of them specify that no data should be deleted for a period of time, posting is disabled on a board, or the entire site so nothing gets deleted.
They also disallow posting of files with embedded data (based on some heuristic) and either strip exif data, or forbid posting images containing it. (the exif stripping thing comes from people accidentally doxing themselves by posting an image from facebook a few years back)
The quality of the fingerprints in the image mentioned is amazingly good, not so much worse than what's produced by proper fingerprint capture devices. The FBI operates a database of fingerprint images (IAFIS) and his fingerprints were taken when he was arrested prior to this incidents and submitted to the FBI (standard practice by nearly all law enforcement agencies). IAFIS includes an automatic matching system which allows you to submit fingerprint images and receive a list of possible matches, and this is also standard practice in criminal investigations (as well as often used for background checks).
So really, nothing about this is suspicious at all, it's completely standard law enforcement practice to obtain fingerprints in any way possible and submit them to the FBI for matching, and then run down any possible matches. Your local police department does this for car breakins if they can spare the resources (not too unusual to get good latent prints off the windows, but it is unusual for police department to spend the time/money). The only odd thing here is LE's fortune, and the accused's misfortune, of images having been uploaded with such clear capture of the fingerprints.
Not digitally embedded as EXIF since it was uploaded to Imgur which AFAIK strip those out, but more like lens scratches and sensor noise, similar to [0].