I think by Lane Assist (LA) he actually means Lane Guidance (LG) as they're different systems. The LA is the annoying one that fires up every time you start the car (but one of the first things I stopped with OBD11 when I got the car), where as LG is the one that's only active with adaptive cruise being on and will do full steering round corners and things rather than just vibrate and try to fight you back in lane when you get close to the edge.
LG uses sensors in the steering wheel to make sure that you're still holding it and those are deleted when you swap from a round to flat bottomed wheel and so the system can't be used anymore I believe. Why they can't, or won't, put the sensors in a flat bottomed steering wheel I'm not sure, but it's just the way they do things.
Hope that helps, makes sense and is actually all correct.