> A database introduces a different data model - I have to translate between database data-types and my programming languages native data-types.

The key concept is explained in this sentence in the article. It doesn't matter how difficult or simple it is; the problem is that it's a different model, so it creates translation inefficiencies and makes it more difficult to reason about.

The object-relational impedance mismatch is a known problem; having data in one place and format, accessed through a single programming model will typically be easier than a separate service with a different language and model, no matter how efficient.