It's not me who finds that OOP is a mismatch with relational systems. It's me and many other people. Ted Neward talked about this 20 years ago. Jeff Atwood talked about it 18 years ago. Martin Fowler defended OOP from this criticism but at least he also talked about it, 12 years ago. There's a Wikipedia page on the subject and many related articles over the years. This is well-trod ground.
Now. Does this mean we critics are right? No. It doesn't mean that. It means that this is well-trod ground. It is the topic of a healthy debate that generates one line of criticism of OOP. Functional programming--which I haven't mentioned in this thread until now--is another source of OOP criticism. What's interesting to me at this particular moment is not the substance of these debates ("is the object-relation impedance mismatch real? is mutable state really bad?") or who's right. What's interesting to me is that author of Sigma's Blog doesn't address any of this. I claim that these two topics--the ostensible object-relational impedance mismatch and the mutable state bugaboo of functional programming--are of such primary importance in generating criticism of OOP that any author who sets out to defend OOP from its critics must address these topics in order to be taken seriously. And yet, all this author says is
It seems like a lot of people dislike Object Oriented Programming
They don't say who these people are, what their reasons are for disliking OOP, or why things seem this way to the author. They just accuse unnamed "people" of not understanding OOP and then dive into a bog-standard description of OOP which is well-understood by all the relevant programmers that I know.
None of that answers your questions on the topic of the object-relational mismatch, of course, but then I never promised to explain that topic to you. I might, but then again I might not. If I don't, there are plenty of resources online that you can study for yourself. If you form a killer argument that addresses all issues once and for all, perhaps you should write your own blog post. Surely such a groundbreaking achievement would be worthy of making it all the way to the front page.