You might be right about the thumb needing to put more flat on the trackpad so that it creates a larger and more elongated shape, because I just made two more videos, both times trying to make the thumb shape larger, and on both times I managed to trigger the gesture only after a few tries:
the weird thing is - it should be that large or larger even without any pressure. Even just lightly touching the trackpad with the side of the thumb should create such a shape.
Probably the reason my thumb has an abnormally small contact area is because I use a soft gel wrist rest next to my Magic Trackpad, which causes my hand to come at an angle to the trackpad. After removing the wrist rest, I can trigger the gesture maybe once out of four attempts:
I wouldn't want you to adjust the four-finger pinch/spread gesture recognition if it would degrade or clash with other gestures' recognition.
Hopefully, macOS Tahoe will still provide an app launcher/switcher that's equivalent to Launchpad and accessible via the system-provided four-finger pinch gesture, so that I won't need to remap that gesture to launching some 3rd-party Launchpad alternative.
Thank you for getting to the bottom of this.