(Author here.) As others have pointed out, I am here referring to SQL as a broad term for "SQL RDBMSes". When I say that it is not a good blob or document storage system, what I mean is this: You can plop unstructured data into a SQL RDBMS using something like JSON serialization, but it's not generally a good idea. Document storage requires flexible schemas, so SQL schemas are also not generally a great idea -- you end up with tables with lots of nullable fields that are usually empty.

As an example, imagine a single database where you want to store actual desktop documents, such as the formats supported by Apache Tika: https://tika.apache.org/1.8/formats.html -- If you try to model this using a SQL schema, you'll likely be in for a world of pain. From a UX standpoint, a user just wants to "search across all documents", but you have hundreds of heterogeneous types with varying degrees of field-level compatibility.