The quote you have provided is not in any way probative because it concerns
Dante's opinion of the "language" spoken in Rome
in his time, and explains why he considered it inferior to Florentine, the Florentine which developed into standard Italian. Nowhere does the phrase "lingua toscana in bocca romana"appear in your quote.
Please provide a source which explains that specific phrase as applying to any deliberate modification of standard Italian by Pietro Bembo or anyone else which then made its way back into Tuscany, which is what you stated upthread.* If it exists, I'd like to see it. If it doesn't exist, then your interpretation is, I'm afraid, incorrect.
Perhaps the confusion arises because you are not giving sufficient weight to the meaning of the phrase "in bocca"? It means literally "in the mouth". It refers to
pronunciation.
*Indeed, Tuscans, in my experience, especially when speaking amongst themselves, do not necessarily aim for either the "Roman" pronunciation or the "quasi Milanese" one even today, and some still display some version of the distinctive "gorgia". You can hear it in the following youtube video.
A young Tuscan homesick for Tuscany because he has been living in Spain writes a collection of Tuscan expressions which he calls a Tuscan "Vohabolario", poking fun at the propensity of people with a strong Tuscan accent for sometimes turning the "hard" c into h.
I think Matteo Renzi can sound pretty "down home" at times.
No less an authority than Claudio Marazzini, Torinese, Professor of Italian at the University of Piemone, Editor of the text "Parlare e Scrivere", and President of the reknowned Accademia della Crusca, one of the arbiters of the Italian language, in response to the statement and question that "It is said that the best Italian is spoken in Florence. How do you judge the language of Renzi, the ex-mayor of Florence?" (and now leader of Italy) not only commented on Renzi's Italian but discussed the birth of Italian.
"L’italiano è nato a Firenze, ma non tutto il fiorentino è passato nell’italiano standard. Certo, il possesso dell’italiano da parte dei fiorentini è sempre entusiasmante. In un bar qui vicino ho sentito la parola “nocchino”, per indicare un piccolo buffetto. È una delizia ascoltarli.
"Italian was born in Florence, but not all of the Florentine language passed into standard Italian."
In my opinion, that's a very precise and accurate answer. I would also add that other language traditions, foreign influences etc., can be seen in standard Italian, as well as just natural evolution through time, and the Florentine dialects themselves have developed since the time of Dante. There is nothing unique about this. It is just part of the fact that languages evolve through time. English today is not the English of Chaucer, or Shakespeare, or even that of the Romantics and Victorians. Foreign influences, especially that of French, have had a profound impact upon it. Languages constantly evolve; if they don't it's because they are dead languages really not spoken by anyone...like Latin.
He then goes on to describe what a delight it is to listen to the exciting and inventive way Florentines use the language. As to Renzi, he has this to say:
Renzi parla un italiano marcatamente toscano, ma non sempre. Come tutti i toscani, il presidente del Consiglio ha certamente delle doti di affabulazione notevoli e lui ci aggiunge una grande capacità comunicativa.
Renzi speaks an Italian that is markedly Tuscan, although that is not always the case. Like all Tuscans, the President of the Council of Ministers has great story telling (or fable making) skills, to which he adds skills with communication.
I wonder how Tuscan other members think he sounds in this video.
https://www.youtube.com/watch?v=7cVRN_RieWM
I quite like the way another Tuscan speaks...Paolo Ruffini... or maybe I just like him. He's completely adorable, especially when the the 60 year old makes him blush.
The segment was filmed in Pistoia. The discussion is slightly risque, so beware if you understand Italian and are sensitive about such things.
https://www.youtube.com/watch?v=oEgLEfmVQF8