I'm not so sure. Flying creatures are classified as such because they have a fly speed, or because the use of a power has granted them one.
The only thing that says a conjuration can fly is single line in the conjuration rules listed under "occupies no space". If that line was written as a separate rule then there'd be no confusion (e.g. Floats: The conjuration does not need to be supported by a solid surface, so it can float in the air).
I'll admit it's open to interpretation though.
FLYING is irrelevant. Conjurations do not fly. That's something
creatures do. I never claimed conjurations can fly.
They are
power effects and nothing more.
So, let's examine the rules hierarchy here.
Can a power effect be placed in the air? Yes.
Does spirit conjuration, or any of its keywords, say that it cannot? No.
Therefore
as a power it has every right to be up there in the air.
Does the rule for conjurations say that a moveable conjuration cannot be moved in the air? No.
Therefpre
as a moveable conjuration it has every right to move up there in the air.
There exist rules stating explicitly it can be up there, and there is a complete absense of rules stating it cannot be up there.
Rules say it can. No rules say it cannot.
This shouldn't even be an argument. This has nothing to do with how creatures fly, or whether it occupies its space. The only thing that is relevant is that a rule says it can, and no rule says it cannot.
Moreover, as you state, there's a rule stating
explicitly that conjurations can float. And spirit companion contains zero words stating it cannot. Zero. Not one.
What does occupation have to do with it? It's not relevant.