It really is quite obvious that an animals alarm signal will make them alert or scare them away. Thats why its called an alarm signal

But really another option if effective is well worth the effort. If you were the farmer and could rig up an automatic speaker to scare the roos and minimise damage it would be great. It would certainly beat being out culling all night.
I know it aint the solve all solution and that culling is still a very serious and practical method. If the roo thump turns out to be cheap and effective then great.