Proper word use.

You can say that.

Here's a little story: A guy has an argument with his wife. He says her mother comes to visit too often. She gets mad. He pulls out the calendar where he has drawn big red X's on all the days his mother-in-law has been there, demonstrating that her mother has spent 40 of the last 52 weekends with them. His wife is now even more angry, and accuses him of never liking her mother in the first place. He runs from the house and finds his friend at the neighborhood bar, and tells him what happened, and says "In theory, I proved my point." The problem is that his wife is still mad and he's still sleeping on the couch. So in this case, "in theory" means that in practice, proving his point didn't really get him anywhere. He didn't really win anything in the end.

The point (not the theory) was the she comes over too often. He did prove his point. But it just got him into more trouble with his wife. It's an idiomatic way of speaking.

If someone says that something is true "in theory" it may have more to do with how it helps or doesn't help than whether the facts are strictly true or not.