I am familiar with well nuts but most of the time you install them from the top and drive the screws into them through the rack from the outside. These are different: the screws enter the rack feet from the inside of the car and have the removable threaded inserts, etc etc. They also do not appear to any "collpasing" to make the seal as the screws are tightened down.

I'm just being cautious....don't want to drill any holes and find out I can't back out of the installation. That's why I'm considering gluing them down with the 3M Super weatherstrip adhesive (the yellow stuff). It's awful strong stuff.