I don't think that's really possible. Whether an action's fcurve is valid depends on what the action is applied to, not on the fcurve itself. For example, assign the cube's action to the armature too. Then the fcurve is valid for the arma but not the cube. Now when you look at the cube in the dope sheet, the fcurve gets a red underline and is_valid changes to false. But switch to the armature, and the red underline goes away, and is_valid changes to true.

If the fcurve is valid for one object, but not for another, a single boolean is_valid can't capture that. The only thing it could do is cache the result of the last time it was checked, so there's no reasonable state before the first check.

In fact, looking at when both the cube and arma are displayed in the dope sheet, I'm pretty sure is_valid just caches whether it got a red underline the last time it was drawn. Literally, the "last time", changing the sort order or resizing the window will change it. So it's not really for answering "is this fcurve valid for this object", which is what you want for T93704.