I'm guessing what happened was that they were monitoring an anonymous chat group on this EncroChat app and weren't sure of the true identities of some of the people in it. They saw one of them posted a photo showing fingertips, decided to see if they could extract and match the fingerprints against their database, and got a hit.
I disagree with the (current) top post in this thread saying it's "likely parallel construction". It could be, but the claimed story also sounds pretty plausible, if it's indeed true that they could match the fingerprints based on that photo.
That said, if they didn't heavily downscale or obfuscate the photo before uploading it, the fingertips look so blurry that I imagine it'd be hard to get anything actually useful from it... (https://ichef.bbci.co.uk/news/976/cpsprodpb/F8F6/production/...). If that's close to the original photo, I would grant more probability to the parallel construction/source protection hypothesis.
I'd be quite impressed if the photo was really that blurry and they still managed to extract fingerprints. Though, maybe it could become more feasible with some Photoshop adjustments?