Except it does...
Now, that is not absolutely definitive of anything, but given the description of a spirit companion and its overall 'creature-like' functionality, it is sure no reach to say that a conjuration IS in fact subject to the rules of creatures when no other explicit rule overrides them.
That statement also covers Flaming Sphere and Mage Hand and we go right back to that. It says that a conjuration -can- be a creature, and that's not in contention. If a specific power says a conjuration is a creature than it is and follows those rules.
However, Call Spirit Companion is not one of those powers and therefore the assumption cannot be made that it is anything but a power just like any other.
Since that statement admits of only 2 possibilities, object or creature, and there are almost no rules specific to objects, we can't really assume there is a difference between a creature and an object either.
A creature and an object are two different things, which is enough. OAs are triggered by creatures, and without a trigger you can't take actions out of turn. Being an object is not sufficient to trigger an OA.
In any case WotC CS has issued a whole slew of answers on the subject. While they are not the final word and as usual aren't 100% consistent the general tone of their answers seems to go in the direction of the SC working like any other combatant in cases where nothing is specified. Thus they have ruled SC are subject to OA pretty consistently for example.
There's one more point against OAs for spirit companions.
Spirit companions do not take actions, they are moved by an effect. They do not take move actions, nor -any- action. Contrast with beast companions which take move actions simultaneously. Compare with conjurations that also can be moved and use the same language and terminology.
Let's face it, the rules exist for having a creature and using powers through the creature, as evidenced by the ranger. The spirit companion doesn't use rules even -close- to that.
Besides -all- that... if a spirit companion were a creature, then it would have a basic attack, as -all creatures have melee basic attack-. Nothing in its description says it does not.... so if it's a creature, where is its MBA?
Creature-like functionality doesn't even apply to a spirit companion. It cannot run, it cannot shift, it does not respond to terrain, it doesn't even acknowledge wall powers that block movement.
It can act as the origin point of your powers, and has effects when people are adjacent to it, and a monster can pop-it if they get a good hit on it. That's all it does. That's not a creature, that's an over-glorified spell effect.