There aren't really that many definitions for OOP; it's a very consolidated paradigm. This is a short but comprehensive guide: https://www.baeldung.com/java-oop
It's simply not true that there "aren't really that many definitions of OOP", much less that the guide you've linked is "comprehensive" when it is specifically about Java.
Academically, you're right. For practical reasons, you probably don't care how Simula, E, Lisp and Smalltalk (languages mentioned in that 20 year old article) implement it. This seemed more like a beginner question so I think the Java definition is a good starting point.