The engine doesn't know what angle it's mounted at. What 40 is saying is by installing it just off by A few degrees it can sometimes help to keep the fuel from putting out the glow plug if you don't quite have the engine in proper tune or the tank draws A little.
Setting things up correctly is the best answer but sometimes you just can't so you can do little tricks to make things work.
I have A nice on board glow driver but I have never had to use it. Some day I may build A plane that for one reason or another will not allow me to set things up perfectly and need it or some other tricks like canting the engine A few degrees to make it run well.
I have an old GP 60 size Aeromaster that was like that but I used the SK .91 two stroke engine in that plane. For some reason this engine isn't too tank sensitive and I got away with it.
Today with all the ARFs set up problems are becoming A normal problem with the cheaper ones so old tricks are coming back in vogue, good things to know.