Methods/sensors for making a "smart floor" to detect human presence via floor?
You seem to have three answers to your title question in the first paragraph of your question. To answer the only remaining question, "do they mean piezoelectric floor tiles?", maybe. This project used force sensitive resistors. 

A much cheaper solution to your highly restrictive situation is to use an accelerometer embedded in the floor. You'll have to test it on site to determine the threshold of acceleration (floor vibrations) that will count as a person in the room. It's slightly more socially acceptable than placing a microphone in the room and then telling people "don't worry, it's not recording your conversations."