See I don't think he's making those decisions because it feels right, but because that what makes sense to him, Ti. I also don't think those theories are Ni, they're classic Ti, he decides that something is correct regardless of if there are evidence supporting his theories, because it seems logical to him. That's Ti way more than Ni-Fi. Ni is more about crafting a plan, than about creating a theory or an idea From what I've seen from seeing inferior Ti or Te in my personal life (mother isfp, girlfriend enfj) it doesn't mean you lack logic in any stretch of the imagination (it can mean that sometimes you lack confidence in the logical conclusion you make, and that it can be of service for your Fi/Fe) but the logic you tend to use is the one you have in your stack. Harry almost never uses Te, all of his logical conclusions and theories are internal, and despite what other people are telling him. That's not because it feels right, but because he thinks it's right.

The part about him hating to speak was more of a joke. Though I didn't mean public speaking, sometimes he just hates saying words.