Obviously, children can hurt other children, but at what point does that become molestation? Wouldn't it be called something else?

I get why you think this, but you're simply wrong.

Definition: "sexual assault or abuse of a person, especially a woman or child."

Basically I think what I'm saying is, that I thought molestation was more strictly defined as what I stated in my post

Defined by whom? Society? The law?

In general, legally, whether or not a child can be held culpable for a crime will depend on a variety of factors, one of which being mental capability. But a ten or eleven year old is far from a toddler, so it's a possibility.

But I don't understand how that would be relevant to your S/O saying he was molested, which it seems like he was.

I haven't brought it up to him or anything

That's probably for the best. I still find it odd that you'd be so focused on trying to find a way to sympathize with his abuser. That's not very supportive or loving.

The girl may or may not be evil. Statistically, it's likely she was abused herself.

And...so what? It's very sad, but that doesn't change what your boyfriend experienced, which, if your description is accurate, is unquestionably sexual abuse.