Can you provide credible references that support your statement as historical studies of phonological and tonal changes in Old (Ancient) and, later, Middle Chinese assertively point to emergence of various Chinese languages that we identify today as Wu, Yue, Mandarin etc to Late Middle Chinese (LMC) (with only exception being Min that had split off from the Old Chinese before Middle Chinese emerged)? In Western linguistics, Late Middle Chinese is identified with late Tang-early Song periods.
Interestingly, Tang imperial annals do, in fact, note already emerging regional pronunociation differences in Middle Chinese across the empire, and, ironically, it was perhaps the first and the last point in the history of all Chinese languages where there existed true dialects of the Middle Chinese language – in the Western sense, anyway.
Even the earliest occurence of what we identify as Mandarin today already indicates the beginning of profound phonological changes, the process that took the next few centuries, namely: a complete loss of checked syllables/contour tones, arbitrary tone changes owing to a loss of nearly all Middle Chinese finals which has also led to an emergence of polysyllabic words in Mandarin: Old Chinese words were nearly entirely monosyllabic and atonic, Middle Chinese words were mostly monosylabic and the tonogenesis had yielded four tones. Conservative varieties of Chinese, e.g. Yue, still have a largely monosyllabic core vocabulary, and Mandarin words, owing to the reduction of phonemic and tone pallettes, are remarkable for having two or three syllables on average. Later influence of Mongolian and Jurchen languages has also had profound effects on Mandarin as we know it today, phonetically and morphologically.
> For example this is how the Song philosopher 朱熹, a Fujian native, spoke in public:
> That's pretty recognizably Mandarin. Even earlier from the beginning of the Tang Dynasty, we have poems that are recognizably vernacular Mandarin.
What distinguishing qualities of the two cited texts make you state your assertions? My paleolinguistic knowledge of Chinese languages is insufficient to discern differences, therefore it is a genuine enquiry on my behalf.
Also, with respect to the Song era, the work «THE NATURE OF THE MIDDLE CHINESE TONES AND THEIR DEVELOPMENT TO EARLY MANDARIN / 中古汉语声调的本质和到早期官话的演变» (https://www.jstor.org/stable/23752830) specifically notes the following (page 187):
3. NORTHERN SONG. Somewhat paradoxically, independent sources for the synchronic state of the language in Song are less satisfactory than for either the preceding Tang or the following Yuan periods. The great Song rhyme dictionaries, such as the Guang-yun and Ji-yun, were revisions and enlargements of the Qie-yun, preserving many distinctions which had long been obsolete. The existing versions of the earliest rhyme table, the Yun-jing, date from Southern Song but the work itself must have originated in late Tang times and its categories are those of LMC rather than the more evolved standard language of Song. Later rhyme tables such as the Qie-yun zhi-zhang tu show some contemporary influence but it is not always easy to distinguish what is merely traditional and what is strictly synchronic. Rhyming in poetry, especially «ci», as opposed to «shi» which continued to be dominated by Tang norms, gives important information but only on limited parts of the phonology.
It would be interesting if you could shed the light on the available evidence (with references) that has shaped your point of view.