I thought cell phones were about 900MHz? - but in any case, the bridge rectifer is to convert the signal to a DC voltage - there will be plenty of stray capacitance to smooth a 900MHz signal.
However, it's obviously not intended as a practical unit, simply as a crude demonstration that a cell phone transmits periodically all on it's own. Notice how close the phone is in the video clip, that's probably as far away as it works.
Certainly you could track a transmitter in this way, it's called DF'ing (Direction Finding) and has been used ever since radio first started. But for a cellphone it's an incredibly crude technique, you would need to be fairly close, and there only be one working cellphone in range. And, of course, you need two readings from different locations.