Part of the problem here lies in the fact that the word theory has two distinctly different meanings.
In science, the word theory is used to denote "a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena".
In colloquial non-scientific parlance, the word theory is mistakenly conflated with the termhypothesis, "a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena". Of course obfuscationists rely on and encourage this confabulation in the public mind to keep the uninformed swallowing their drivel.
The latter is the mistaken sense in which the term is used when people utter the platitude, "just a theory". The former, scientific meaning is what is actually indicated when the phrase "peak oil theory" is used amongst those who understand the phenomenon. Denialists of course attempt to convince people that the colloquial misapplication of the term is what is meant...