Doing a handstand pushup against the wall is not 10 times easier. Free standing is a bit harder than against the wall. But a person than can that easily do handstand pushups against the wall will have little problem learning the balance to do the same free standing.
Also your bosu ball example is just terrible. Against the wall vs free standing is akin to comparing dips on a bar to rings. Sure at first you can barely do them but in less than a few weeks time after you practice you can knock out as many as you do on bars. What is a lot harder is headstand pushups vs handstand pushups, the difference is huge.
A good example is a person who can do a freestanding headstand pushups (head goes to floor) is still ways away from doing freestanding handstand pushups (full range of motion), but a person easily knocking out feet against the wall handstand pushups (full range of motion) can learn to do freestanding in less than a couple of weeks if not days.