the maximum file size is smaller than 500MB so that's a moot point and not many people are going to be hitting that size limit in the first place... it is a password manager after all.
Not that performance or file size would matter in this case, BUT what using SQLite would allow is to use a single format for persisting all aspects of the password database, with immediate, programmatic, random access to all fields, including attachments.
But I also agree, that for this specific use-case, even SQLite is a bit of an overkill probably.