Once you have multimethods, you don't need to couple the method definitions to the class body. But you're still doing object oriented programming; CLOS is an object system. The fact that people assume that OOP==Java is unfortunate, but doesn't change the fact that CLOS is also OOP.
Beyond that, I think you're conflating how method bodies are placed in a file with "belonging to a class". I can define C++ methods outside of the class definition, but surely no one would say that such methods don't belong to a class.
Well if 95% of the population use a term to mean one thing, then arguing that no it really means something else doesn't get you anywhere.
And no, I'm not conflating those. In C++ and Java, classes are, among other things, namespaces. When you declare a method in a class, it ends up in that namespace. Where it is defined is unimportant.
When someone says "object oriented" unless they define their terms, or are clearly coming from the smalltalk school, I assume they are talking about C++/Java style OO, since that assumption is right the most often.
This is particularly true when someone is talking about "what's wrong" with OOP
Beyond that, I think you're conflating how method bodies are placed in a file with "belonging to a class". I can define C++ methods outside of the class definition, but surely no one would say that such methods don't belong to a class.