The alcohol is just used to thoroughly clean & prep the surface before sticking any adhesive (double-sided tape from a spoiler) onto it. Alcohol also dries/evaporates fast, so as soon as you wipe the trunk lip area down you can apply the spoiler onto it immediately afterwards.... assuming that the surface is dry and that you didn't use too much alcohol where you still see it damp or wet on the trunk.
Just like painting something, all of the work to make the paint job last is done during the prep. If you do a lousy job prepping the surface before sticking on a spoiler, then it's probably going to start coming off the car later on (like most factory Performance spoilers on Model Y and Model 3 cars). Sure the curve of the spoiler also has something to do with that, as not all spoilers are made exactly the same. But for the most part, if it has good double-sided tape stuck to it and you prep the trunk lid surface well, then it will stay in place. I'm sure Tesla doesn't do much prep work when installing these from the factory and just wipes the trunk lid down with a rag so it 'looks' clean, and then just quickly sticks on the spoiler (not using any painters tape to hold it down and let it set) and then the car moves onto the next station at the factory.
I just like doing some additional things to help insure that a spoiler won't come off or start to raise on the ends, like using 3M adhesive promotor on the trunk where the tape will stick to, and using a hairdryer to heat up & soften up the double-sided tape so it's more pliable when sticking it to the car. Also, using some painters tape to secure the spoiler will help keep the pressure on it and let everything set. You obviously don't need to do these things, but your spoiler will stay on better if you do.