Inasmuch this conversation is subjective, I just do not think that statement is objectively true. It is a basic OO principle that the outside world is not privy to the inner workings of an object (or class if you prefer).
Whether you like that or not is immaterial. You are free to avoid that aspect of OOP or the paradigm altogether.
It's called the Uniform Access Principle, as I've already mentioned in a separate comment. Getting back to your example, it is not very useful without mentioning a language under examination. There is no single definition of OOP, and every language implements in a slightly different way.
There are languages where UAP is enforced by the syntax (eg. Eiffel, Smalltalk, Ruby) and languages where it's a matter of convention. But even in the latter it's considered a good practice to keep properties private and encapsulate them with accessors. Some even have dedicated features that help with that (auto-properties in C#, properties in Python).
> I haven't studied OOP extensively in a strict setting, but I would have said that in the OO world, a property is completely separate to a method.
There is absolutely no reason for that, and the grandparent of all things object-oriented (Smalltalk) very strongly disagrees with you: everything is a message, and there are no properties.
> A property defines data about an object, and a method defines something you can 'do' with the object.
A getter method is a property and a method. Properties are irrelevant, access should be uniform, the implementation details of an object (whether a "property" provides direct access to internal data or not) is of no concern to the user of the object.
A property is nothing more than a method whose contract is "give me some data"
There's OOP qua Smalltalk/Kay, and then there's OOP qua Java. You're describing OOP qua Java, which is an ugly type of OOP designed for certain performance advantages.
Whether you like that or not is immaterial. You are free to avoid that aspect of OOP or the paradigm altogether.