The Apollo 10 LM was indeed short-fueled, but it wasn't because that fuel was unnecessary to the mission's goals: proving out the basic profile of descent, ascent, and rendezvous in lunar orbit without the added complications of the first landing. In addition to the short-fueled state of the LM, there was also the minor fact that the ascent stage was overweight by a fairly significant margin, meaning even if they'd had full descent tanks and had disobeyed orders to land, their return from the surface would have been impossible. This is ignoring revisions that would have had to have been made to the guidance computer, to the equipment stored on board.
Basically, Apollo 10 was not loaded properly for a moon landing because it wasn't going to be, it was going to be a test flight to prove the last critical hurdles before the first landing. In order for it to be used for a lunar landing, many, many changes would have had to have been made with the specific goal of making it a landing mission, it would not have been simply a matter of filling the tanks all the way and then disobeying orders. The spacecraft was not mature enough for the mission on that flight.
