Complex sentence: They were called the "old texts" because they were written in the pre-Qin writing, and were only transliterated into the new style writing a century or so later than the new texts.
Split into simpler sentences: They were called the "old texts" because they were written in the pre-Qin writing. In fact, they were only transliterated into the new style writing a century or so later than the so-called new texts.