If the machine is just launching rocks at a specified angle and velocity, then no it is not computing a parabola. The rock's trajectory traces a parabola, but it has not been "computed" (in my view). A computation to me involves an explicit mathematical result related to some specific intent or question, not just physical objects playing out the laws of nature. The computer would have to make a statement, a result - "based on my calculations, at time X the rock will be at position Y."

Then you would clearly reject all analog computers, since e.g. an analog adder just involves electrons playing out the laws of nature. A rock launching machine is an analog computer for calculating a parabola. A specific result might be "where is the intersection with some Y value, and you have a platform at the height for that value. The distance the rock lands at would be the X value.

What is the point of reducing everything to nothing, or to the same thing? I mean I get it, technically it's valid, but it's very boring, there's nothing then to talk about. We are humans and we do have subjective experiences and classify things into different groups, and trying to understand the relationships and differences between groups gives us interesting things to think and talk about.

But hey, if a rock and a computer are the same thing, and computation is meaningless, words are meaningless, etc - why have you bothered to say anything at all?