Ahh that makes sense.
I currently have an external SSD with the beta on which I boot to periodically to check if things work and when Rocksmith kept crashing I thought it was an issue that needed Ubi to fix which most likely wont get done now as they want to flog you RS+ with its "amazing" library, glad its just a custom DLC patch issue that hopefully will get fixed in time by community. Luckily the Mac isn't my main Rocksmith machine so its no big deal for me if it doesn't work for a while after Sonoma gets released.
That being said when I did the file verification and Steam downloaded the original files again and the game loaded I did see the game mode icon in the menu bar which I really hope improves performance on my base M1 MacBook Air as right now on Ventura it seems to suffer with random slowdowns now and again, but when I look at the system resources the GPU is sitting at its base clock and nothing is being taxed in a way that would cause slowdowns so I'm guessing a Rosetta issue but hopefully Game Mode may also help when Sonoma is released which is what I really wanted to test with the beta.