I gave it all additional thought and came to conclusion that Coppola underrates Tom.
He is not a man to be played so easily, to swallow everything in it’s straight meaning and sitting tear eyed being called a brother. Impassive-faced Tom, who never trusted anybody’s word, always looking for double meaning, calculating and figuring everything out!
Tom, who lived and worked so many years with Vito, and Vito, as we know, never told anything straightly and entirely! And he perceived every subtlety in Vito’s mood, every hint concealed behind his words! Tom, who never displayed his emotions without thinking things through, who analyzed everyone around and was pretty manipulative himself!
He becomes in FFC’s hands naïve and snotty somewhat like Fredo…