No where in that article does it say no penetration. The article's focus is on the fact of the very obvious unstimulated oral scene. But does not mention anything else. Do you really think the actors would voluntarily offer that info. Especially, if no one asked that particular question. I have read many articles/ interviews on this movie. I have never seen one that the Actors or Director said explicitly that any of the sex was simulated. Go check for yourself. Since the scene is filmed with their genitalia mostly in the dark. I don't think anyone thought that they could be doing it for real. So no one asked them. Some might say a case of missing the forest for the trees.
Since we are quoting articles.
What Patrice wanted was to capture the desperation of Jay and Claire's lovemaking. These intense sessions were called "the Wednesdays" and would punctuate the film, being different each time.
We are, of course, fascinated by what goes on in other couples' privacy. Their bodies, thoughts and conversation are compelling. They were for us as children and continue to be so. However, I can't help wondering whether sexuality is better written than filmed. Looking may be more erotic than reading; it is more immediate. But looking may also fail to capture the intricacies of feeling; it won't necessarily increase our understanding. In fact all it might do is make us embarrassed or conscious that we are watching a choreographed sexual act; it might merely make us feel left out.
Perhaps this is because of the way sexuality is usually portrayed on film. Patrice and I talked about keeping the camera close to the bodies; not over-lighting them, or making them look pornographically enticing or idealised. It will be a sexuality that isn't sanitised, symbolised or bland, that isn't selling anything. The point is to look at how difficult sex is, how terrifying, and what a darkness and obscenity our pleasures can be. Patrice will, therefore, have to make a sexually explicit film. To a certain extent the actors will have to go through what the characters experience, which will be difficult for everyone.
https://www.prospectmagazine.co.uk/essays/56451/filming-intimacy
Reading this article one could infer that the sex was all real.
But again believe what you want. But personally, after brighting the video as much as possible, leveling out the vibrance , desaturating the color. adding back a little blue color, zoom in and watching the video in slow motion. And with the lack of artifacts that effective area in question. Honestly, can't believe that there is a question. I ask that you all do those steps and then tell me what you think.