Also the guy didn't say he was middle class, he said he FEELS like middle class in some ways. Those two things are pretty different.
It's because his life still has OR in it, and he's allowed himself to believe there's such a thing as a lifestyle that doesn't.
When a person is really poor, they might have to choose between rent OR food. With a bit more income, they can have rent AND food. At the middle class level there's enough AND to cover the needs, some wants, and a few nice-to-haves. But not all of the wants or nice-to-haves, because those are still OR decisions.
People fantasize about having a completely AND lifestyle, where they don't have to make tough decisions or give up things or experiences that they want in order to get others. But it's just a fantasy. Nobody ever truly gets to a complete AND. It's a desert mirage or the end of the rainbow. No matter what dollar figure people think they need, when they attain it they might have the pot of gold but the end of the rainbow is somewhere else. Why? Because the AND is not complete, and new OR options have appeared out of nowhere. They always do. Resources are finite, but options for consumption is not. Potential expenditures always appear to consume and then exceed all means that you choose to make available.
No matter how high your income becomes, there are always tradeoffs to make. There's always a lifestyle that's even more AND-ish that would allow you to exercise both options simultaneously if you so chose. That I think is what it means to "feel" middle-class the way the author stated.
There's such thing as a happy, relaxed life, but a person simply needs to stop chasing the complete AND. That's what happens when some billionaire decides to give it all away, or when a Mustachian goes FIRE. They are satisfied with the level of AND they have.