I forget, this may not apply to people who pitch on clay. I can absolutely see how where one places their foot--regardless of being closed, straight or open, that it would hold position. Pros pitch off an extremely firm surface. That being said....99% of the pitching world does not enjoy firm, clay mounds.
I pitched almost exclusively on dirt where the rotation of your upper body during follow through combined with landing leg spikes in the ground over powers the dirt underneath you and your foot turns a bit to the glove side. Every single mound I see these marks. I'm not talking about crop circles. This is not an X-files like phenomenon. It exists. Even if someone lands straight on, their foot slides forward a bit, too. This is the dominant condition of nearly 100% of youth baseball field mounds I have encountered.
You only need to compare a youth mound to a pro mound after a game has been played on it, to see that these mounds do not hold up. The ground is literally shifting under your feet with every pitch. I was using this image to illustrate that the plant foot certainly lands closed for a lot of pitchers and the evidence of that fact is found in the marks from the landing foot that start with toes pointing toward the batter's box and not the target.
EDITThis is not a critique for Zach Lutz or his mechanics. I'm just pointing out that the very first video I watched after responding on this thread...I see the landing foot rotating to the glove side during the end of the follow through. He's pitching without spikes in a driveway. There is no way for him to firm up. This is what youth pitchers face--even on the field with most mounds. That foot motion creates the crop circles I'm referencing