Isn't it obvious? If you can't do arithmetic without a calculator, it makes it hard to do algebraic manipulation. (Un)luckly a calculator that could do that arrived in my lap just in time. Then you get to calculus, and it was the same story. I eked by with a lot of furious typing, but the mental workload of that made it untenable towards the end and I wasn't really gaining much anyway because of that. It would have been far better if I'd just hadn't been allowed to use a calculator from the start.

It didn't affect my ability to reason and prove things, just as long those things don't strongly require the knowledge and skills I should have gotten from the calculator shaped gap in my education. I lack a lot of the background knowledge and/or familiarity and comfort with many skills that I should have.

I think we're using the term “mathematical ability” in different ways. For me, it's an ability to prove a theorem, not to solve a differential equation.