A proven fact takes a lot more than 10 years of post production decline in the UK (see Russia). Does this mean that I don't think the UK and US have peak, no of course not - because where things cannot be proven beyond doubt we rely on common sense and logic based on empirical observations etc. Russia could have been deemed to have peak well back but logic and common sense points to other reasons for its first production hump. In the UK and US common sense points to a true peak but that is not proven fact - and to claim so gives ammunition to all those trying to (unsuccessfully) discredit the probability of a near term peak. Claiming to have proved something that is essentially unprovable discredits the argument.
Bandidoz, I agree that Peak Flow would probably be a better term and more understandable to most people when first explained.
Theory is however the correct word in a scientific context. I agree with you that some people may misinterpret the word through naivety, but I don't think using an incorrect word because some people deliberately use it to obfuscate. Using a different, and less correct, word would just lead them to obfuscate slightly differently and possibly, due to an incorrect word being used, actually have some right.
The word theory has a number of distinct meanings in different fields of knowledge, depending on their methodologies and the context of discussion.
this is what Wikipedia (ok probably not the best source) has to say about Theory.
In science, a theory is a mathematical or logical explanation, or a testable model of the manner of interaction of a set of natural phenomena, capable of predicting future occurrences or observations of the same kind, and capable of being tested through experiment or otherwise falsified through empirical observation. It follows from this that for scientists "theory" and "fact" do not necessarily stand in opposition. For example, it is a fact that an apple dropped on earth has been observed to fall towards the center of the planet, and the theories commonly used to describe and explain this behaviour are Newton's theory of universal gravitation (see also gravitation), and general relativity.
In common usage, the word theory is often used to signify a conjecture, an opinion, or a speculation. In this usage, a theory is not necessarily based on facts; in other words, it is not required to be consistent with true descriptions of reality. This usage of theory leads to the common incorrect statement "It's not a fact, it's only a theory." True descriptions of reality are more reflectively understood as statements which would be true independently of what people think about them. In this usage, the word is synonymous with hypothesis.
"You can't be stationary on a moving train" - Howard Zinn