First, I want to say defensiveness is not a scum tell.
Moving on, Nesso's thought process (or as I see it, at least) is that he was trying to gain reactions from his scum reads. Michelle reacted unreasonably and a bit out of character, Nesso perceived it as a slight defence of gambit and joby, while still explaining to Michelle that his way of sorting people also works.
Your gaining town credit is...well. He was referring to joby and gambit as scum and gaining town credit, not town gaining town credit.
And everything out of there is pure WIFOM and you are biased. Nobody accused joby and gambit except for you, which was a post ago. It's a very thin line to walk on, you're basically saying he's scum for being defensive about something that was really out of character and uncalled for Michelle. He wasn't being defensive because his buddies were outed lol.