There is this line in "To the Reader":
The books are divided into chapters, for the ease of the reader, who may thereby the better pause and breath when he thinks fit; and to supply a chronological table in the ten last books, the distinction of times is observed in the notes, both by the olympiads and the Christian era; for the relations in the first five books were long before the olympiads began, and the history is so ancient, that the certain times of persons and things there related are, for the most part, unknown or very uncertain.
I believe that's a translator's note, not a quote from the original text.
The "To the Reader" part is from the editors/translator. If you read more of it, that becomes clear (it specifically discusses translating Diodorus into English).
The language sounds old fashioned because it's a 200 year old translation (which is why it is public domain).
Are you reading the George Booth translation, which is out of copyright and available online? I believe you are reading Booth's introduction, not the text from Diodorus Siculus. If you look at the very next paragraph he talks about the translation and what fragments are included in the book.