I’ve done a lot of research into HIPAA (I work in a dental-adjacent field) and my guess is that it’s almost certainly the latter – an assumption, maybe based on something they were told. But it’s still on them regardless of whether they were deceived or simply didn’t ask.
There have been very few dental practices who have paid fines for HIPAA violations and one that stands out is one who hired a document shredding firm to destroy old paper patient records. The shredders pick up a bunch of files and just drove around the corner and hucked them into an open dumpster where they were found. The dentist was fined as the result of their assumption that a document shredding firm would, you know, shred documents.