Does searing meat lock in moisture?
No. As you noted, searing beef performs what's called the Maillard Process (or Reaction) which is a specific form of caramelization. Nothing is "sealed" into the meat because the meat isn't sealed by the process. It's still porous and will therefore leech moisture during cooking. You can retain moisture in cooked beef by buying quality beef and not cooking it beyond medium-rare to medium.