I assume it has onion-skinning based on this picture (from the clickstore page). Those red shadows look like onion skins to me (although they seem pretty much identical to the main figures, which is a bit weird):
As for tweening, I guess it depends on what you mean by tweening. Back when I used Macromedia Flash, it had 2 types of tweening: shape tweening, and motion tweening. In both cases, it would use the vectors from the head and tail frames, and interpolate the in-between frames accordingly. In the case of shape tweening, it would actually morph the shape of the vectors, whereas in the case of motion tweening, it would merely move/rotate/scale them, keeping their fundamental shape unchanged. In both cases, it would essentially create the in-between frames for you, and you could then edit those frames yourself if you wanted.
My guess is that Spriter might not have that kind of tweening, where it actually creates in-between frames for you. Actually, having lots of animation frames is expensive (RAM-wise), and I think one of the purposes of tools like Spriter is to reduce the amount of actual graphic frames you need. To achieve this, I think Spriter uses the concept of bones, which is similar but different to tweening. So you "rig" your character up with bones (1 bone for upper arm, 1 for lower arm, 1 for neck, 1 'bone' for head, 1 'bone' for hair, etc.), and then you can tell Spriter to move the bones however you like. For example, you tell it to start the leg in a back position and end in a kick position, and it will perform the in-between animation states for you automatically. If it supports Inverse Kinematics (which I think it does) then it'll go even further: if you tell it to move character's fist from next to its ear to straight out in front of it (ie. a punch movement) then it'll automatically straighten the elbow and adjust the upper-arm and lower-arm in a realistic way, without you having to do anything to those bodyparts yourself. But all if this is likely to take place programmatically, at runtime. It won't actually create any PNG-based graphics within the Active Object that you can view in Fusion's animation editor.
I want to stress again that I've never used Spriter, so I'm just speculating based on the screenshots I've seen and on how other animation tools I've used do it (I use Toon Boom Harmony). But I guess my point is that it might be worth your while to look into Spriter's "bone" features, though they might not work exactly how you had in mind.