I haven't actually read the book, but from what you posted it doesn't sound that weird.

Of course, it's hard to know word for word how people spoke (out loud) in everyday life, but I have some examples from letters and recorded conversations. Like I said, I haven't read it, but from what I've heard it takes place in France during the revolution, so I'll use examples from there.

Here's a translated letter written in Paris in 1796. It's not exactly an example of a casual conversation, but it's a good example of very sincere "speech" at the time. There are two texts in the link, but the first one is a joke, and written to sound ridiculous and pretentious.

Here is a translation (not mine) of memoirs written in the 19th century by a woman who lived through the revolution. It's obviously not a word-for-word example of what people actually said, but it was written by an eye-witness, who must have thought that it was at least likely that something like that was said. Some examples:

“My God! How imprudent I was to think of him! How I would blush, mother, if you knew my weakness! How I would deserve to be scolded by you! But how unhappy your daughter was! I was in love and I wanted to hide it from you.”

I then indeed saw my mistake and I wanted to leave him at once; but he insisted emphatically that I should stay, seeing the ill he had caused me. He said to me: “Good Élisabeth, I have caused you much pain, but forgive me for it. Yes, I admit, I wanted to know your way of thinking. Well then! The one I prayed you to seek for me, dear Élisabeth, is you: yes, my friend, you are the one I have cherished since the day I first saw you. I have found her therefore, the one who I had been seeking everywhere! Yes, my Élisabeth, if you want, I will ask your parents for your hand this evening; I will pray them to make for our happiness right away.” He took my hands then and said to me: “But you do not respond? Do you not feel for me what I feel for you?”

I was so seized with joy that I could not reply; I believed myself dreaming. He was still holding my hand and begging me to respond. God! How happy I was! I told him then that if my parents consented to our union I would be happy.

He pressed my hands tenderly and told me: “I love you too; fear nothing; you’re dealing with a good man.” – “Me too, Philippe, I’ve loved you since the day when I saw you at the Convention with Charlotte, at that evening session… I still have your lorgnette.” – “And,” he said, “I still have your ring; it has not left me since the day when I fell ill and could no longer see you. My God! How I suffered, deprived of the news of you which was so dear to me for so long! No longer being able to hope to see you again sometime with Mlle Charlotte, all these thoughts were far from advancing my recovery. I wrote to you ten times a day, but I did not dare to send you my letters, fearing to cause you chagrin, good Élisabeth. Several friends came to see me, but no one spoke to me of you; judge of my distress! Finally Robespierre came one day; he was the only man from whom I could have had new of you; but how unhappy I was! I did not know I how I could ask him. Finally, it occurred to me to speak to him of his hosts; he praised the entire family most highly, spoke to me of the happiness he felt to be among people so pure, so devoted to liberty. I already knew this from several of my friends; but, my Élisabeth, he did not speak to me of you. My God! How I suffered for many days. This time was so long… Robespierre the Younger came at last to see me. What joy for me! I was more familiar with him: we were of the same age. We spoke of his brother. Finally, I could no longer restrain myself; I spoke to him of your family, of your sisters; I spoke to him of you, my Élisabeth. He praised you, told me that he had the friendship of a brother for you, that you were cheerful and good and that I liked you best of your sisters, that your good mother was excellent, that she had raised you well, as housewives, that your household was perfect and recalled the golden age, that everything there breathed virtue and a pure patriotism, that your good father was the most worthy and generous of men, that his whole life had passed in goodness. He told me that his brother was very happy to be among you, that you were a family to him, that he loved you like sisters and regarded your father and mother as his own parents. If you could have known, my Elisabeth, how happy I was to hear him speak thus of a family I already honored, and whose conduct toward Robespierre, toward the friend of liberty, had made me recognize and esteem! I wished for the return of my health in order to be able to meet you like in the past with Charlotte…"

(different part)

Finally, I heard my father call our good friend: he was so good that we loved him better than a brother. My father informed him of the subject of the conversation and told him: “My friend, it’s our Élisabeth, our scatterbrain, that M. Le Bas is asking us in marriage.” – “I congratulate you on it,” he replied, “so much the better. Élisabeth will be happy; my dear friend, don’t hesitate for a moment: Le Bas is the worthiest of men by all accounts; he is a good son, a good friend, a good citizen, a man of talent; he’s a distinguished lawyer.”

That good Maximilien seemed happy to see me asked in marriage by his compatriot and pleaded in our favor with my parents; he added: “This union will, I believe, make for Élisabeth’s happiness; they are in love; they will be happy together.”

He praised me and my good friend; my mother made a few more objections on my distractedness; but our friend assured her that I would be a good wife and a good housekeeper.

It was almost one in the morning when he retired to his room, wishing my father and mother a good night. I then heard my father say: “There is no reason to hesitate after the way Robespierre has just praised his friend.”

Our good mother loved her children equally; she feared, in marrying her youngest daughter first, to harm the eldest; my father thought otherwise and said: “If they are in love, must we delay their happiness! No, wife, we must put prejudice aside and consent to this union.” My good mother appeared disarmed then and said to my father: “Well then! My friend, until tomorrow; he will come to ask for your consent.” I heard no more speaking and went to bed, but quite sadly, for I feared that some difficulty would arise.