Yeah they're argument is based on saying that sniffing a user agent string is illegal reverse engineering. If they get the right 100 year old judge they might even succeed but it feels like a thoroughly lame argument to me.
According to the video about this by Louis Rossman, there wasn't even string sniffing. No changes were made in the code, the client ID was hard coded in, and was untouched by the author.
It's like putting up a sign that says "No trespassing, unless you know the secret code word, which is 'Stegosaurus'".