I wonder if locals have always known about the tomb, but never realized it could be of interest on an international level.
These site inventories are generally filled using cultural resource management records submitted by surveyors, miners, construction companies, etc. who are often legally required to file them. A few tour guides I’ve used in Mexico found new ruins in the jungle and submitted their records with GOS coordinates and pictures. If locals knew about it, someone likely recorded the location a while ago.